SMS is part of the service frame - every time your phone talks to the cell tower to go "here I am, I am IMEI 12345 and I have sim ABCDE!" there is a 160-character frame for it to send a SMS message, and every time the tower goes "hello IMEI 12345/SIM ABCDE, I see that you are in cell ZXCV!" to track your equipment's location, there is a 160-character frame for it to send a SMS message to your phone.
Well, we can't just trust your phone that its plan is valid, so there is also a service frame where the tower says "Sorry 12345/ABCDE, your plan is inactive!" that uses the same frame... and it has a 160-character frame for SMS too.
SMS is literally free for providers to implement, it is just an inherent part of the phone's ping/pong process of talking to towers. So there is certainly no requirement for an active plan of any kind. As long as your phone is on the network it is notionally capable of sending or receiving an SMS, the provider just won't let it... but usually service messages ("your plan is inactive, go to this website to top up!") will be allowed.
This is on top of emergency service - 911 calls (or local equivalents) will work regardless of plan status. Actually I'm not even sure you need a SIM card at all, or if that can be done simply by IMEI...
Basically: just because a sim doesn't have an active plan, doesn't mean the SIM or the phone isn't active itself. There is still information interchange happening, and that carries SMS frames.
But you don’t have a phone number. Who does the sender send the text to? How exactly this work in other countries where an inactive account can receive a text?
This is all very confusing because you're mixing up your definitions of what precisely is "inactive" here.
It's like the OSI model, there are multiple layers here representing different things. An IMEI is a representation of piece of equipment. A SIM is a representation of a subscriber (or, to be more precise, it's a cryptographic 2fa token that a subscriber carries), so a piece of equipment may have multiple SIMs and a subscriber may have multiple SIMs each associated with at most 1 piece of equipment (at a time). A SIM may be associated with a phone number, and may be associated with a plan which may be active or inactive.
The data model is really:
user <-one to many-> SIM token <-many to one-> IMEI
And a phone number is an at-most-one feature of a particular SIM token.
The fact that you didn't pay your bill this month doesn't mean your phone number is inactive - someone who dials that will get a "call cannot be connected" message because the phone network still knows it's you. The carrier just chooses not to connect your call, the phone number is still actually mapped underneath.
And even if your number eventually gets reallocated, the fact that your SIM doesn't have a number associated with it is irrelevant - the network still knows you by your SIM and knows your phone by its IMEI.
The phone number is really like a domain - it's a human-readable abstraction for the physical reality of the routing layer (SIM/IMEI). And the SIM is a representation of what user-token (a user may have many tokens, but a token has at most 1 user) is using a particular IMEI.
At the network level, they don't care about your phone number - that's just used for a "DNS lookup" of what equipment needs to ring. And they can send a message to that equipment even if there's no actual phone number associated with it. You can also have a IOT SIM where there is no actual 9-digit phone number to ring it (although that's a US-specific routing scheme, other countries do it different) and the network just talks to it via its SIM.
And even if you don't have a SIM (subscriber-token) the phone still talks to the network, and can still make e-911 calls and similar, you can initiate outbound traffic too, because your phone is still connected to the network even if there's not only no phone number, no plan, but even without a SIM. It's still an IMEI in a cell talking to some particular tower even if there's not a SIM in it, and can both send/receive metadata traffic or even real traffic (e-911 calls).
I'm probably getting the finer details wrong here too... it's a very complex model with a lot of entities and relationships.
For some fun tangential stuff on the topic, especially surrounding the SIM card, check out this DEFCON video. It goes into the 2fa nature of the SIM - actually the SIM is a full security processor (javacard) that can execute arbitrary javacard applets sent by the network, and push/poke stuff into the SOC or baseband directly, it is like an "Intel management engine for phones" and it has a huge amount of power over what the SOC can do and see on the network.
The phone has a SIM card, and a phone number, as evidenced by the fact that he "did not pay it for a while" rather than never having had a plan. So the phone is active and can connect to the network and authenticate with a base station and attempt to make outgoing calls. Those attempts will be rejected by his phone provider.
Incoming texts will also be rejected and not delivered. In other countries (I know about Ireland, anyway) this isn't generally true.
Receiving messages is free, so as long as you have a prepaid card that hasn’t been deactivated (not using it for a year is usual with my provider) you can receive text messages without paying.
If I were on a prepaid plane, and I didn't top up, my phone would still have an active SIM and phone number. I would still be able to receive calls and texts.
It dated to mobile calls being expensive especially outside of a very bounded home area and being either a business expense or a relatively luxury thing. In that context, it didn't really make sense to hit a landline with a big charge for calling a mobile number--perhaps unwittingly. And that general thinking carried over to SMS.
As somebody else outside the US - where is this different? Without a connection you can only make emergency calls, receiving anything is impossible when your phone doesn't have a number without an active sim card.
Except they did have an active SIM on a prepaid plan, that hadn't been topped up. In a situation like that I would still be able to receive calls and texts, just not make them.
What country do you live in, and what's the process for being assigned a phone number that can receive text messages without paying anyone for a phone plan? Does the government have an office where you can show your ID and get a SIM card for free?
1. Buy a sim card from ALDI for 10€
2. Activate
3. Use the phone until the balance is down to 0€
At this point you've got an empty balance, are unable to make calls or send texts, but you can receive calls or texts still just fine. You can also visit zero-rated websites (in the past e.g. 0.facebook.com).
When I was a child my parents would give me a phone with a SIM card in this state, they could still call me if they needed to tell me to come home, and I could still call the emergency services, and given WiFi I could surf the open web as well, but I couldn't waste money on paid services or calls.
