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Ask HN: How can scam callers fake a mobile phone number?
141 points by fxtentacle on March 14, 2022 | hide | past | favorite | 150 comments
I'm with T-Mobile and I just received a phone call on my mobile phone from another number where everything except for the last 3 digits was exactly matching my own number. I found that suspicious, but I was curious enough to pick up the call. The other person greeted me with "We are very important this is Interpol!" in seriously broken English, so I suspected a spam call and hung up to try to call them back. That didn't work because the phone number they were calling me from does not actually exist. Like I immediately get the T-Mobile announcement informing me that this is an invalid number.

Now I am wondering:

- How can a spam caller call me with a source phone number that does not exist?

- Shouldn't my mobile phone network verify that the caller - which was also inside their network - is a valid subscriber? Otherwise, how can they bill someone for this call?

- How does this kind of scam call work technically?



The how - depending on the protocol.

Signalling System No. 7 - ISDN User Part spec (found here: https://www.itu.int/rec/T-REC-Q.763-199912-I/en) allows you to specify both a calling party number (3.10) and generic number (3.26) (the UK spec adds an additional presentation number so you have 3). This will typically require the help of an operator which is 'connected' to the network on the PSTN. A real business case can be made; like a generic, non geo support numbers appearing on the persons phone instead of the geographical number of the office which called. Either a bit of social engineering or findings a less scrupulous operator is all you really need to do

SIP has FROM and P-Asserted-Identity headers which follow the same process


Thank you for this rabbit hole. Today I learned a lot about modulation, frequencies, and how DSL works :)

In the end, the most surprising snippet of knowledge for me was that Erlang (that Amazon S3 is built in) was invented by Ericsson for live patching ISDN phone routing systems without dropping any ongoing call.


I do not believe S3 is written in Erlang... do you have a source for this?


SimpleDB is; maybe they confused them.


Sorry, yes. They called it Simple Data Service = SDS in the article and then I incorrectly assumed that it's what we now know as S3.


Does that addition in the UK spec add any extra protection? I've never had a spam call with a spoofed number in the UK as far as I'm aware, and definitely never seen that thing that happens in the states where a call comes in showing the name or company name of the caller, even if they're not in the receiver's address book.


The UK spec adds a few fields which I can not remember all off the top of my head. One addition is the 'presentation' calling line identifier, which is screened like the 'calling party number'. The generic calling line identifier is not screened, hence the addition

ref what the screening bits are: https://www.dialogic.com/webhelp/csp1010/8.4.1_ipn3/exsapi_q...

As for if the UK & spoofing, it's a very real thing with very real business cases and abuses.


Thank you.


Caller id is just a user settable field. There are two numbers, ANI which is how telcos are supposed to keep track of who to charge. NO one uses it, because users don't like it. And caller id is sent out on the second ring, but again, user can set that to anything. Corps have to adhere to the TCPA, others don't and SIP calls are cheap and globally routable. https://www.fcc.gov/sites/default/files/tcpa-rules.pdf


That's a ridiculous system design.


It’s remnants from a time where security wasn’t a concern. The original intent of the From: field in email was that it’s definitive, but now it’s just a legacy field that many systems ignore because it’s fakeable.


Is it because earlier there was no display of numbers (ex in rotary phones)?


Possibly, but I'd wager more that it was due to Bell owning the entire network, so they knew that any "from" numbers were correct. If I'm reading Wikipedia right, SS7, which is still supported today, was created all the way back in 1975![0] Bell wasn't broken up until 1982.[1]

[0]: https://en.wikipedia.org/wiki/Signalling_System_No._7

[1]: https://en.wikipedia.org/wiki/Breakup_of_the_Bell_System


You're correct, but back in the day, all nodes were trusted nodes, so would have been a lot of overhead to authenticate all this stuff. Hasty regulatory oversight in a fledgling industry led to the current situation.

STIR/SHAKEN actually has the potential to do things correctly, as a call Digital Attestation Certificate has to be supplied... but telcos make quite a bit of money off of scam callers so don't expect them to move quickly, and I'd expect them to implement it in the absolutely poorest way possible.


Similar to the original SMTP implementation, it was designed in an era where folks often assumed that only "trusted parties" had access to the network backbone (whether that network is circuit-switched or packet-switched).


Brought to you by Ma Bell


It is actually incredibly easy! If you are using a voip line, it is just a configurable field in the UI. You can do it with any voip phone app (e.g. [1]) and a voip provider (e.g. [2]). I have an old archived video showing it here [3]. It is not so interesting though, just me poking around in a voip provider's UI.

To address the other question about phone providers verifying stuff. SHAKEN/STIR [4] protocols are supposed to address this, but I think the telcos are still in ramp up time.

1. https://www.zoiper.com/

2. https://voip.ms

3. https://odysee.com/@cybering:1/spoofing-call-id-using-voip:2...

4. https://www.fcc.gov/call-authentication#:~:text=STIR%2FSHAKE....


It's ridiculous that phone companies allow this. Anyone wanting to set caller ID via voip should be forced to provide some sort of verification that the number is theirs and the phone company should not route it if it fails verification.

We only have 3 major cell carriers here is Switzerland, it should be trivial for the 3 to verify each other's numbers to see if those customers even exist. Unlike the US each cell provider has his own number prefix. Numbers are portable but only between certain providers.


Welcome to the wonderful house of cards that is the SIP protocol


The iOS feature for silence unknown callers has really been a blessing to my life. I have screenshots of me getting ~10 blocked calls per day. I turn it off when I'm expecting a call, but it saves me so much distraction and rage about my vehicle's extended warranty.


It's not as easy... For example, it is possible and legal to use your own number to call from a VoIP provider, so the recipient can call you back on your actual phone.

On the other hand, it should be possible to detect at least a percentage of spoofed caller IDs and block them (e.g. non-existing numbers).


The VoIP provider could forward the call to your phone as a middleman, or there could be 3 numbers(1. Who to bill 2. Calling number 3. Reply-to number) and only #3 is user-configurable.


The idea of using a VoIP provider would be to _avoid_ using one's phone, e.g. to get better rates... And the reply-to number, it's unfortunately not the way telcos work... and changing all the infrastructure for this would be very very expensive.


> Shouldn't my mobile phone network verify that the caller - which was also inside their network - is a valid subscriber?

Since the advent of number portability, the area code and prefix no longer signify anything about what carrier a particular number belongs to. You could very easily take your T-Mobile number to Verizon, for example.


The number itself is no longer permanently their network, sure - however, they have instant access to look up* any number and find who presently owns it - which they must use to be able to route calls successfully. It's completely true that say, T-Mobile could today refuse to connect a call when the caller pretends to be calling from a T-Mobile owned number and yet is calling from outside their network.

Sure, it would only kill a subset of the spam if they could only do this when they currently control the spoofed number themselves, however it would still do something!

* Source: https://teraquant.com/local-number-portability-and-how-to-tr...


STIR/SHAKEN is supposed to stop the spoofing. For an explanation, see https://en.wikipedia.org/wiki/STIR/SHAKEN


The problem is that carriers (especially smaller ones) have been dragging their feet on implementing it, and nobody can use it to actually block calls until essentially everybody supports it and is interoperating. Until then, phones will just show calls between STIR/SHAKEN carriers as having verified caller ID.


would it be possible to tell my carrier to simply block all call that are not (STIR/SHAKEN)?

If all my friend are on carrier that support it, I am not interested in receiving call from people that are not on a carrier that support it.


I highly doubt any carrier would offer this, especially with the current adoption being where it's at. You'd be better off using your phone's capabilities to restrict calls to only your contacts.


This is what I do, and feel is the only solution now. Phone always on DND/only allow contacts to ring. A whitelist approach if you will.


This is great and all until there’s an emergency and someone is trying to reach you.


Yeah, it's less than ideal, but this is the future that the lazy carriers have brought us to. Hopefully, someone trying to reach me in an emergency would have the brains to send a text.


You can do this in software on your phone (assuming Android).


How does one do that? Do I need another app? I don't see a setting for this in my Android settings.


In the Phone App, under Settings -> "Spam and Call Screen", there are bunch of Spam and Call Screening options.


There's also an app called YACB which offers advanced filtering.


I think the smaller carriers have till middle of next year to implement STIR/SHAKEN.


Isn't it already possible for a phone to display the STIR/SHAKEN Caller ID verification status of each incoming call now?

This would be useful in the interim as this system rolls out, and would also encourage adoption by mobile carriers


> The Federal Communications Commission requires use of the protocols by June 30, 2021

Spoofing still exists, though. Is the issue now that our phones are backwards-compatible with the insecure system?


That deadline has been repeatedly extended


STIR/SHAKEN hasn't worked correctly or stopped robocalls like promised. Congress basically told everyone that this was the answer and would stop robocallers for good, but in reality did barely anything at all. The real solution is to label robocallers as terrorist and sanction countries with large amounts of robocalls for sponsoring terorrism. Before long, everyone will be too scared to even consider working in a robocall center and they will start turning on each other and reporting their bosses. They should even offer monetary rewards and protections for providing intelligence on the people operating these.


And if they do not comply, to follow through with a Hellfire missile, right? /s


If someone and the coworkers on their floor are calling and scamming old people out of their retirement money, then there is no sarcasm needed here. That should be perfectly justifiable.

I'll paint the targeting laser myself.


I mean, think about how many people scammers are killing prematurely as is. Once these countries that we previously let shit on Americans for a long time have some serious sanctions, I'm sure they'll find ways to deal with the problems themselves.


I live in Canada, and I and most people I know receive spam calls from spoofed numbers on a semi-regular basis.

Sometimes the number only a few digits off from my number, but other times it has a name like TOLL FREE SERV. A common lure is claiming they are Service Canada or Canada Revenue Agency (or the nonexistent Revenue Canada), and the call will open with nonsensical threats like “A warrant has been placed in your social insurance number”. I have a hunch they often target wealthy international students, as sometimes the messages are entirely in Chinese.

Recently I received three calls in one day. It’s been happening for years, and the phone companies don’t appear to be able/willing/motivated to stop it. Most people I know have just resorted to not picking up calls from unknown numbers.


If you are on Telus or any of their other brands you should be able to configure "Call Control". This feature prompts an unknown caller to type a given digit before the call is actually connected. Filters out 100% of all spam and other automatically dialed calls. Very very rarely I had people miss the prompt and as a result not being able to reach me. We're talking like 2 calls in 3 years. They reached me via email and all was good in the end.


I live in Canada too, and these particular calls are rapidly destroying the notion of answering a number that is not in your contact list.


on iOS, the "Silence Unknown Callers" feature has completely eliminated this for me.


Canadian physician here: yeah, this is hell for us trying to reach patients through the hospital system, which is "unknown" by default. Straight to voicemail and you can't really leave a message. Totally agree though, this problem is very time consuming, if only for the time it takes me to look at my phone and decide not to answer the call - a few seconds of my life each time. My worst day was a few days ago; 7 calls.


You're going to hate me then: I go a step farther than OP: Do Not Disturb mode 24 hours a day, 7 days a week. No notifications, no messages, no phone ring at all. I make sure my voicemail is never full. Leave a message and if it's important I will get back to you.

If I have a sick relative and explicitly expect an urgent call, I can easily and briefly turn off DND mode.

If the concept of a telephone never existed, and "Phone App" was invented today, it would be considered extremely intrusive and likely not (at least on iOS) pass App Store review. Think about it: Here's an app that allows any random person to cause your device to 1. interrupt whatever foreground app you have running with system-level UI (notification or full-screen takeover), and 2. ring and vibrate your device without your consent. If we weren't already familiar with telephones, we would never accept such an obnoxious app!


That is just the android facebook app in lite mode! I tried it for about 24 hours once after much social pressure. Now I just look at my wife's feed if she mentions something interesting.


In the US, I've had several problems with physician clerical staff calling me on their personal cellular phones instead of using an office line with proper ID set, then upset because they could never get hold of me. No, I don't answer calls from random strangers, and I don't do answering machines or voicemail, which seems to be something so radical some people have trouble understanding the concept.


Developer here, the cost is more than a few seconds. That call can derail productivity for much longer, like pushing a heavy stone that must gain momentum


Out of curiosity - what do you mean by "Can't really leave a message" ?


Not the OP, but if you are a doctor and need to ring a relative or a friend to inform them about a very sick patient, leaving a message without the opportunity for at least brief interaction isn't something particularly pleasant for either party. ( You either leave too nonspecific information that not being sure what to do, or too much that potentially is breaching patient confidentiality or is going to panic the recipient)


Ah that makes sense, thanks. I wasn't sure if there was something I didn't know about blocking unknown calls on iOS or Android that also prevented voicemails


I was actually waiting on a call back from a parcel company, so the scam caller in this case just had very lucky timing.


Because the design of the original caller-id system allows the initiator of the call to attach any set of numbers they like as the caller-id value that is shown on your phone.


Which is hilarious because the phone company used to charge extra to bring you this information as if they were telling you who was calling. That's a service I'd almost be willing to pay for today if it actually worked.


Which is also why it is simply an "initiator settable field". When "the phone company" brought out the service (for a monthly fee) there was only one "phone company" and so they could be assured that they themselves were setting the value to the correct source.

Now that the phone network looks more like the internet (many different companies all exchanging "calls" with each other) that decision, way back then, has the unintended side effect of allowing the robocall spammers to set whatever set of ten digits they like on their outgoing calls.


One thing people don't know is that the phone network is actually a bunch of duct-taped technology that is pretty old. There has been advancements, and if you're in the US, you'll be happy to know that mobile carriers require stir/shaken handshaking, which is _mostly_ equivalent to https on the web (this is a gross simplification).

The short/simple answer is carriers don't care, because they make money when a call is placed on their network. There is also a difference between what is a valid number (digits are correct) vs a real number (someone owns a number). It is cheap for a carrier to check validity, but not "realness" - to check a real number, a carrier may have to do some sort of data request to any number of carriers to determine if the number is owned.


Because carriers are arcane companies that have a monopoly on a swath of infrastructure and have little care or perspective on what products and consumer experience amounts to.

They buy and install equipment and sell out the voice/data.

They actively oppose, thwart any kind of thoughtful innovation, competition etc. on anything relating to their networks, because they believe they 'own' the network and therefore 'own' everything going on on top of it.

Remember the 10-cent 'WAP' pages? Tiny, crappy, useless little mobile web pages? And they wanted 10-cents each?

Carriers would originally not sell BlackBerry service. They thought it was stupid to have 'email' on their networks. BlackBerry had to buy data and then sell to the C-suite.

Then, BlackBerry literally became the reason that people wanted to buy data. The carriers then said - you can't buy network and resell it, you must sell your products through us.

Imagine if some private companies controlled all of the roads. Any and business wanting to put a car on the road had to pay a toll, and the owners could decide which kinds of cars, when, and for what reason and intervene. They tried to provide the ambulance and transport for everyone and keep messing it up.

It's also an artefact of human organization, even a fairly enlightened community/government body would have difficulty setting clear and appropriate guidance.

The issue becomes problematic when there is a control of a scarce resource.

In truth, it's absurd that people should be able to easily fake 'from' numbers, we should have fixed that a decade ago.


Being able to set outbound Caller ID is something that is common with SIP providers and T1/PRI providers. The most common case today is using SIP. The billing happens at the provider level, and is not based on the user defined Caller ID field. Anyone can setup an Asterisk instance and make the caller ID value on the outgoing calls whatever they want [1].

[1] https://www.voip-info.org/setting-callerid/


I just got a call from the Microsoft Security Team. They informed me my computer was highly infected. I spent 1 hour with them executing all cmd commands they wanted & told them the output.

In the end i told them my wifi was broken and the technician should come by soon to fix it. She turned very aggressive and told me to call my brother Internet provider right now, as this is urgent because the hackers are already in my system. I told her to call me again the next day.

I might have forgot to mention i am using a mac (and had to google the result of all commands & screens). I wanted to setup a VM and trace them or maybe even let them execute a manipulated cmd.exe to create a reverse shell. But after my attempts to buy some time so i could set everything up, they gave up and never called again.

So sad, i am still scared of all the „viruses of very dangerous hackers“…


You'd probably enjoy the content of Jim Browning, a guy who tries to flip the tables on these kinds of tech support scams: https://www.youtube.com/channel/UCBNG0osIBAprVcZZ3ic84vw


> I just got a call from the Microsoft Security Team.

They used to call be, but they said they were from 'The Windows'. I tried to get them to play Zork, but they weren't very interested, and it took me a little too long to get it started anyway.


I always figured that the ability to set an arbitrary phone number was a feature for the benefit of large corporate PBX systems. Every person at the company gets their own phone number, but the number of physical connections to the phone company is limited. The PBX can set the identity on an outgoing call to match the phone number of the person who initiated the call, no matter which physical line it uses.


Worked in business telephony, this is correct.


Back in the day you used to be able to spoof caller ID using in-band signalling. it was like a few fun sounding handshake tones and some static-sounding data that you would play after the official one, and that was called orange boxing. That was the Bell 202 FSK signal and I remember hearing it on landlines up until a decade ago if the phone was picked up as soon as it started ringing.

In Canada caller ID also includes the name along with the number from Nortel equipment, while in the USA it's just number. Nobody I know has a landline anymore except for businesses because if it's just the odd crazy person who still makes a super annoying life-interrupting phone call, more than half of calls are just fraud shit with spoofed caller ID and everything. It's so easy you could get started doing it yourself with freepbx installed on some 5$ VPS within minutes. Honestly we need better telephony systems, but everything is being completely superseded by chat apps anyways. Again only crazy people give me actual phone calls anymore and I have two lines between two countries.

Fun things to do to the fraudsters: Talk really quietly and when they are like 'sir i cannot hear you' put yourself on speakerphone and YELL into the phone as hard as you can, and you win the game when you can hear them rip their headset off in ear pain because they turned their volume up to hear you. Either that or ask them what they're wearing until they get mad at you and call you homophobic things.


Again only crazy people give me actual phone calls anymore

I was going to say "Wait!", but then realised all my calls were from recruiters and HR departments.

So I guess you're right.


For me they always want to set up a zoom call nowadays


Since you are on T-Mobile verify you have scam id and scam block enabled: https://www.t-mobile.com/support/plans-features/self-service...

> which was also inside their network

A phone number isn't like an IP address, the call isn't coming from that number and almost certainly didn't originate on the t-mobile network

The FCC recently reduced the amount of time some companies have to implement STIR/SHAKEN to June 30, 2022.

https://docs.fcc.gov/public/attachments/DA-21-1593A1.pdf

>The Commission recently shortened the extension for a subset of small voice service providers likely to be the source of illegal robocalls.


Signalling system 7 has no authentication.

That's the bottom line.

Adding authentication is pretty obviously not trivial, not just because of protocol upgrade issues, but also because end-to-end authen. won't be easy to add at all, and hop-by-hop authen. w/ something like "egress filtering" won't work in the age of phone number portability.

What might work is a TCP-like return routability test. I.e., have the network ask the ostensible device "did you mean to make this call?", though that might have other issues (think of how SYN spoofing can be used for DDoS attacks).

I.e., preventing caller ID scams is really hard.


Here in China they aggressively egress filter since ~15 years ago (source: had an E1 on fiber way back then). You can set caller ID to any number you are assigned and nothing else.


Yes, this would work. It would have to be implemented in a lot of telcos. It would take time to get it all deployed, but there's no better time for this than today.


What about charging a penny or 5cents per call? Nominally cheap for regular users, would put a dent in scammers. And don’t let the phone company keep the money, put it towards the infrastructure.


Billing is one of the highest costs for telcos.


Caller ID is for "presentation" only, not billing. Anyone with the appropriate access can set their caller ID to whatever they want. Some VOIP providers don't do any validation that you "own" the number you are providing. Years ago, when I had an Asterisk PBX set up using a super cheap SIP provider, you could put anything you wanted in for a caller ID.

There are legitimate use cases for this. Imagine if you are a company with 1000's of physical locations. You want them all calls to appear that they are coming from the corporate headquarters.


https://en.wikipedia.org/wiki/Caller_ID_spoofing


I think a much better question is, why can scammers spoof a phone number? We hear lots of excuses from the carriers about how this is out of their control, this is how the system works, etc. Why don't they feel like they have a fire lit under their asses to fix the issue?

My immediate guess is that they must make money off of scam calls somehow. A scam call is still a call.


The same principle is applied when you use call forwarding on most SIP providers. They "spoof" the real caller's Caller ID when forwarding the call, so that the forwarded call reaches your phone with the number of the actual caller, not some random number assigned to your account or the provider. If they didn't do this you wouldn't be able to call the original caller back directly from your call log.

This would be a legitimate use case for Caller ID spoofing.


Yeah, no, not buying it. I mean, not buying that's the only way it could be done.


Yeah, and the net effect is that they're shooting themselves on the foot: I don't even pick up phone calls anymore. And I don't think I'm alone in this: most families and friends claim only to answer calls of close contacts. Legitimate services nowadays just contact you in WhatsApp instead.


This is why I like having my number from where I lived in 2005 (see https://xkcd.com/1129/ ) - any calls from "my area code" are automatically spam unless it is a particular number I know.


This is the same thing for me. Short version is that I'm from Detroit area, lived in NH for a while, and now reside in Tampa area. My cellphone has a 603 area code (NH). But since I'm not from there, the only 603 numbers I care about are already in my address book, any others are guaranteed to be spam calls. This lets me easily ignore 95% of them (I also occasionally see an 802 (VT) number or a 978 (MA) number).

What is interesting is that I have started to receive more 727 (local area code) spam calls, maybe 2-3 per month. I suspect this must be from local friends and contacts leaking my number through sharing address books with various apps.

I am at the point now where incoming phone calls are near valueless, other than from a very small set of numbers. Most people text me, or contact me through other apps/methods. Even for business purposes, incoming calls are almost always scheduled and the very very few that are not, and from an unrecognized number, can leave a voicemail.

It is somewhat amazing how the telco's have let their core product, voice calls, become nearly worthless by not handling these spam call problems. Now I'm using contact methods and apps that are not provided by telcos and not strictly reliant on their networks.


I get scam phone calls with area codes from most of the major metro areas all over the USA although since it's a California area code, it seems most are from somewhere in California.

Your final paragraph is pretty on-point.

> It is somewhat amazing how the telco's have let their core product, voice calls, become nearly worthless by not handling these spam call problems

I have more than once thought about just giving up on having a phone number at all. Unfortunate that isn't an option yet.


Ported my "2005" number to a voip carrier, and have an automated lookup - not in my known caller list? Straight to voicemail. Known caller? Forward to my direct number with a known caller ID, otherwise muted.

I should not have to put this much effort into not being contacted, but otoh, it saves me quite a bit since dropping the US cell line.


I have this convenience as well. If I don't have a number from my home area code, I don't need it. Although I basically do not answer any unknown phone numbers at this point unless I'm expecting a call from a company for something.


Decades ago, an American company called DAK used to sell a box that picked up on an incoming call (land line, pre-cellular era) and played a short message, usually something like "Please enter your extension number" or "enter security code." If the correct code was not entered, it simply hung up. No ring, no answering machine, no hassle.

The nice thing was, you could pass your phone number out to everyone, but it would only ring for the people you gave the code to. And it was easy to manage, just give your number as "555-1212 ext.382" or whatever. And if the code got spread too widely, you could just change it and give anyone you wanted to hear from the new code.

I keep hoping someone will make an app like that for cellular phones, but most people seem to like saving their spam calls in their voicemail boxes to review them later.


I wish phone calls came with a verified (cryptographically) description of the route a call took to get to me. Then I could use a library/app to filter by source or by bad actor (providers that lie about the route). That would enable services like UBlock Origin, allowing for user-generated blocklists.


Great idea!

Local area number but traceroute to a call center in India? Automatically reject.

That'll also solve the issue of unusable corporate phone support because they outsourced it to save a few bucks.


I hate to sound like a conspiracy theorist, but it's pretty easy for a carrier to determine devices that are making spam calls because they log everything, and they could simply create and distribute apps to their customers to enable reporting of spam calls, but somehow for years they've left it up to dodgy 3rd party app providers and the calls keep rolling.

I don't think carriers have any incentive to stop spam calls because they gain a lot of money every year in billing minutes for those spam calls (mostly prepaid accounts are affected by the billing unfortunately)...

I wouldn't dare go as far to say that the calls are possibly even sponsored or conducted by profiteers in the game... (People who sell prepaid and metered phone services)

Just a personal opinion though.


> Ask HN: How can scam callers fake a mobile phone number?

International Telephone Standards. VoIP VoIP Companies like https://www.sipgatebasic.co.uk/tour

And if you set up a VoIP number and a pbx like freeswitch or asterisk, they will send the ringing tones down to the caller so if you have the pbx set to record calls you can listen to what the caller is chatting about whilst they are ringing you, hearing the ringing tone at their end waiting for you to pick up. All a bit spooky but thats the technology for you!

> - How can a spam caller call me with a source phone number that does not exist?

Again they have the VoIP number but when you ring it they can play a dead line tone down to you instead or a ringing tone. With VoIP and Freeswitch/asterisk and probably other PBX's you control all of that.

> - Shouldn't my mobile phone network verify that the caller - which was also inside their network - is a valid subscriber? Otherwise, how can they bill someone for this call?

Depends on the telecoms standards in the country and/or the telecoms provider.

> - How does this kind of scam call work technically? Any member of the public can set up VoIP number and PBX's like freeswitch and asterisk and do this.

If its not a VoIP then telecoms companies and the security services in your country, or maybe you mobile phone is hacked and your mobile has logged onto a local fake cell instead which is slightly different to the VoIP setup above but I dont know how much this device can do. https://en.wikipedia.org/wiki/Stingray_phone_tracker#Active_...

and you can do things like this https://www.wired.com/2010/07/intercepting-cell-phone-calls/


The easiest way is to have a SIP gateway that uses a too long number to display. Usually it's around 12-13 digits for the subscriber number depending on the country code, so all digits before that (after in SIP) will be cut out on most phones.

I think the relevant spec for that is E.164 which enforces 15 digits overall (1-3 for country code and 12 for subscriber number).

There are also lots of SIP gateways that have an ISP license or a phone provider license. They're the same types that allow to fake the numbers for their customers, and usually you can transfer some still in use mobile numbers to them as well. Because apparently law enforcement doesnt do anything against them.

And yes, never use 2FA via SMS. Never.


Saying no to 2FA SMS is a little harsh. It’s strictly better than 1FA password. What you probably mean is don’t use SMS for account recovery.


> - How can a spam caller call me with a source phone number that does not exist?

The same way they make a call with any source number. The two source numbers in a call (ANI and CallerID which don't need to be the same) have historically been not required and not validated. See stir/shaken for a modern effort to change this. Coming soon to a carrier near you; maybe.

Being able to set the source number enables many useful things as well as some spam/harassment/fraud uses. It requires a lot or coordination to allow the former and restrict the latter.

TLDR: don't trust caller id. Don't call people back unless you know the number/it's an expected call.

> - Shouldn't my mobile phone network verify that the caller - which was also inside their network - is a valid subscriber? Otherwise, how can they bill someone for this call?

Call billing records don't use caller id in the way you're thinking. If you pay for incoming calls, they're charged regardless of the source number, but it's recorded for informational purposes.

For outgoing calls, the call record is made closer to the source and is tied to the line that made the call, not the source number.

For intercarrier calls (which almost certainly the case here), the source carrier bills its customer and the interconnecting carriers count minutes on calls and settle up for net difference in flows (calling carrier pays, but interchange fees are going to zero among US carriers)

> - How does this kind of scam call work technically?

Get a phone account where you can set the caller id and calls are cheap; call a lot of people; successfully scam one or two; take the money and run.

Some voip accounts let you set caller id. Traditional primary rate interfaces (T1) usually do too.


To go further on this, the T1/DID allows you to set various numbers for the outgoing (for example, so that all calls from your company appear as "main company number", or all calls from support people come from the "support number"). The CallerID is very easy to replace with anything, but even the ANI can be replaced, and until recently, nobody verified anything at all.

And lots of "back end" things depend on this silliness - for example, some MVNO actually have TWO phone numbers associated with the phone: a VOIP "real number" and a secret "actual cell number" - Republic Wireless had this for sure. The VOIP number is what you'd give everyone, and they'd do routing weirdness to use Wifi whenever possible. The "real" cell number would go direct to the phone but not normally appear anywhere.


Yeah when I was on Republic Wireless I'd sometimes get calls from people who had called the "secret" number because it had been recycled. I used to get calls from the county clerk's office reminding me of my upcoming court dates and probation appointments. I called them back and said you must have a wrong number, they checked and of course had no record of my phone number on any of their records and could not understand why I was getting these calls, nor could they figure out who should have been getting them. Later I realized that someone must have had that "secret" number recently and it had been recycled into Republic's pool.


I don't know the answer to your question(s), but if you're curious, you can download an app right through the play store to fake your number. I used to prank my friends all the time.


Used to take about 5 minutes to configure an Asterisk [1] PBX, obtain a provisioned DID from a VoIP provider and set your outbound caller ID with Set(CALLERID()) [2]. Doing so allows you to configure both your text label and call-back number.

[1] https://www.asterisk.org/

[2] https://www.voip-info.org/setting-callerid/


Do you know how e-mail lets you set anything you want in 'From' field and only relies on optional stuff like DMARC to, maybe, verify it?

It's almost exactly the same with phone calls, that 'From' field is just set at a provider level instead of user level - and there are many providers over the world, including some that allow the user to set this field however they like.


If you hang up be sure that you have really disconnected the line. https://bc.ctvnews.ca/beware-of-the-delayed-disconnect-phone...


This should be an easily solvable problem. All calls should come from a paid account and be trace-able to the payee (by the phone company). I don't get why there is so much phone spam. If we need new standards, let's get on it!


seriously, never pick up the phone unless you know the caller. every stranger who calls you is trying to waste your time in some way

even 'legit' businesses that call you from random numbers are basically a spam channel / are training you to get phished -- for example health insurance and credit card. every time I call back on their official # to ask what they want, it's 10-20 minutes to figure out what they wanted (if they even know!)

we somehow aren't a society that can legislate to prevent spammers from using the phones. at this point let's pivot and punish legit businesses who use the phones to waste my time


They are doing ANI spoofing. By using a service they can show you any number you want. The law only states that you can’t do this if you are trying to commit a crime.


Try https://assistant.dasha.ai/ to block such calls.


paris hilton knew how to do it back in 2006, checking lindsey lohan's voicemail by pretending to call her from her own phone

https://www.infoworld.com/article/2658949/paris-hilton-accus...


It still seems crazy that even though carriers knew how easy it is to spoof numbers, even back then, they still decided to just skip any voicemail passcode authentication if you were calling your own number from your own number to get to your voicemail.

It's like letting someone in your house because they're holding up a paper cutout of someone else's face that you know in front of their actual face and that's good enough.


Just never answer the phone. If they really want to talk to you -- they'll find another way.


Or they'll leave a message.

The real answer to the problem is to deprecate the legacy telephone system. It will never be as secure or user-configurable as just about any modern implementation of voice/video over IP.


>deprecate the legacy telephone system

The legacy telephone system is being deprecated. All three US mobile operators now have VoLTE (ENUM) interconnection with each other. STIR/SHAKEN call verification is happening between the mobile operators and large consumer VoIP operators like Comcast and Charter. VoIP is far cheaper to operate than POTS and most all operators are using it now and shuttering their legacy networks.

The issue has to do with regulations around the phone system. Rural call completion to small operators is a requirement - as it should be - but has loopholes that encourage abuse. This rural call completion regulation also comes with the ability for small operators to charge certain prices for call completion so they can afford to keep their high-cost rural customers serviced. The larger carriers pay these fees to connect their subscribers to those of the rural carrier.

However, some smaller operators have also been using these higher rates as a means of profiteering by allowing massive amounts of spam traffic through their networks towards the larger carriers.


Thanks for the educational response! I've been long wondering why it seems as if carriers are implicit in the widespread scam.


Hoping this is something that doesn't need to be said here, but just in case:

This is why you should NEVER provide personal information over the phone if you didn't initiate the call. It doesn't matter if your caller ID says it's your doctor's office or your bank or whatever.

Hang up and call them back at the number you normally use to reach them, from their website or the back of your credit/debit card for example. Make sure you're talking to the people you think you are.

Otherwise they can phish all kinds of info out of you.


Also, if you were rung on a landline, ring back on your mobile.

I'm not sure if it's always the case, but I believe that a call to a landline only terminates when the caller hangs up. This certainly used to be the case.

This allows scammers to ask you to hang up and call them back on the number on your card (for example), but they just mimic the dial-tone and ring, then they have another scammer answer the phone.

This is not an issue on a mobile.


That depends on where you live. Some areas never had it. Some had it but got rid of it, some still have it. Check with your local phone company is the only way to know what currently applies - though they can change the rules anytime as this is just a setting in their switches (they may or may not also have legal oversight here, again depending on where you live).

Or of course as you said, always call back from mobile.


Or if you can't use a different line or mobile phone, intentionally dial a different number, if it rings through to "the bank" then you know they have hijacked your line.


Ah yeah I remember that trick. If you didn't hang up after talking to a friend, you'd could "haunt the line". Works best if someone is waiting to use the only phone in the house. Which is a phrase you don't hear that often :D


Ah, a ghost in the wire.


I haven't seen a landline in ages...


They are buried.


Ah, puns. I dig it.


I remember many years ago getting a random phone call from someone who claimed they were a detective investigating a case. The detective sensed my skepticism right away over the phone and suggested that I look up the phone number for his police department, and to ask for badge number ZZZ when I call. I called back, everything was above board, they came and interviewed me (they wanted to see if I still had something in my possession which I thankfully did), they made a mark on the item to indicate that they had seen it already and I never heard from them again. Though I wonder if they ever caught whoever it was that was using that item ...


You can't drop a story like that on us and not tell us what "the item" was ;)


I found a video of the GP:

https://youtube.com/watch?v=3VlyZIywY9c


All this could be faked, this could have been an elaborate scam.


Yes, if the stakes were large enough, & the adversary sufficiently skilled/resourced, it is possible an impersonator could also intercept & redirect your call to "the police" – for example by compromising phone network systems or the police-department's switchboard. (Or, somehow corrupt your usual means of looking up that police number.)


How could they fake the receiving end of the call of a legit precinct number without some massive resources lol?


This.

A while ago, my wife got a call from a collections agency on my phone. They asked for her name, I asked "which one?"

She said "I can only talk to ____."

I said, "I understand but which one would are you looking for the older or younger?"

"I can't share that information."

"Then I can't put you in touch with who you're looking for if you can't tell me who you're looking for."

"I can only speak with ____."

I said "Ok, tell me the last 4 of her social and I'll know which one you're looking for."

"That's private information, sir."

"No, that's the public portion of a social security number. Tell you what, since you're learning how this works, I'll make it easy on you: I'll give you the first digit and you tell me the last. That way we both know we have the same 'protected information'. If I give you the wrong starting number or you give me the wrong ending number, we both know we have the wrong person."

"I need to speak with _____."

I gave her the starting number but she didn't budge.

I finally said "okay, since you can't verify who you're looking for, I'm going to just tell you you have the wrong number. This is my cell phone, not ____'s. You may send a letter, but this is not the correct phone number for who you're looking for. Please, do not call me again."

While all this was going on she gave me the name of the collection company which I was able to Google and determine it was a legit operation and located not too far away. A medical provider never got our correct address, but it just showed me how overly trusting some companies expect people to be. Nah, this is a two-way verification. If you're gonna call me, you need to give a little to prove to me you're legit.


> No, that's the public portion of a social security number.

It really isn't.


Indeed, the first three are dependent on the state that issued your SSN, and the next two often are sequence bits roughly correlating to when it was issued. If anything, those are the public bits


They are not numbered by state anymore


The change was done in 2011, so very likely the geographical-based system applies to anyone fielding collections agency phone calls in the present day.


True, I'll give you that


It is, and you must set aside your personal feelings and treat it as public info.

If people think these digits are like some secret password, they will be treated as such and used to gatekeep access to even more restricted info and accounts. Which would be a disaster because many people have had these last four digits exposed over time. Knowing them does not prove identity.

They are public.


Like it or not, many banks and other industries use it as a verification question. A collections agent is going to be explicitly and strictly forbidden by policy from disclosing it to an unknown person on the phone, and that's a good thing.

Yes, I'm pretty sure everyone has my SSN and other details, but collections agents should absolutely follow the rules. The ones that don't tend to be abusive.


You might as well assume your entire SSN is public. Most of them have been leaked someplace or other, and for anyone who was an adult before about 2000 it was common to have them pre-printed on your personal checks. It's only in the past few decades that they have suddenly become "secret."

That someone who called you on the phone happens to know your SSN last four or even the entire number should not confer any trust on your part.


The second you believe your SSN represents privileged information, that’s when you get taken advantage of.


Regardless of original intent, at this point, it really is.


The last time I received such a call, I just told them in Hindi to not to call me again, and that I knew it was an obvious fake. I could hear them muttering something in Hindi on the other end while hanging up.

The tongue-rolled accent & peculiar pronunciations / choice of words is a giveaway. A large number of these IRS & collections call originate from call centers in India.


> A while ago, my wife got a call from a collections agency on my phone.

I occasionally get calls on my number from a caller asking for my brother.

My standard response has always, from the first time one came in, been: "there is no one at this number by that name".

> You may send a letter, ...

If one /ever/ gets a call from anyone purporting to be a "collections agency" then this ("send a letter") is the /only/ correct response that should ever be given. You may want/need that legal record later.


I once sold a house while in a different state, and the real estate agent was positively baffled that I wanted to know how the person who was supposed to collect my banking information and identifying information would identify himself to me.

They really just expected me to meet some guy in a polo shirt at a coffee shop and let him take a snapshot of my passport and all my bank account info.

Frankly scammers have it too easy when the real thing works like this!


It can even be a problem if you do initiate the call. My mom got taken in by a scam where she googled "American Express phone number" and a scammer's website or paid ad (she isn't sure which it was) was showing at the top of the results with the wrong number.


Yet another example of the often discussed "Google search results infestation"...


Also possible if they gave malware on the computer, and it modifies search results in the browser. That’s always seemed like a serious vector although I haven’t heard much in the way of in the wild exploits.


Computer malware + phone scam sounds like a remarkably targeted, technically broad, and labor intensive attack.


It is, which means it targets higher net worth individuals. I just listened to a darknet diaries episode where they talk about how much these attacks cost to execute - some are $10-100k per target.


that's why you only call the number on the back of your card if you can help it. and double check that you entered it correctly


> Hang up and call them back at the number you normally use to reach them, from their website or the back of your credit/debit card for example. Make sure you're talking to the people you think you are.

This is not foolproof either. In some older landlines even hanging up doesn't necessarily disconnect you.

This means an attack works like:

1. Attacker dials their victim, alleging to be "Interpol", "VISA Card Services" or some other similar thing.

2. Victim takes this advice, "hangs up" and picks up and dials back.

3. After victim hangs up, attacker plays dialtone noise down the line, which they have not disconnected.

4. Victim picks up and "dials" the actual thing they want to be sure of, but is really just listening to a fake call the attackers play to them.

5. Attacker answers "Thanks for calling X".

This isn't to my knowledge true of mobile calls but it's important to know it's not foolproof either.

There's some discussion of that here: https://security.stackexchange.com/questions/100268/does-han...


So many callers get offended when you try to tell them that you aren't going to give you personal information to some random caller unless they can validate themselves first. And many times their direct call back number (if they have one) isn't the one published to the company website. I get the feeling that most people don't question them.


If someone can't prove who they are, I don't care if I offend them or not.




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