On traditional landline phones that older folk tend to use (although I guess that's dwindling every year) a common tactic for scammers is to ask the victim to call the number on the back of the card.
However the scammers don't hang up they just play a fake dial tone so the victim dials the number thinking they're contacting their bank but they're actually just speaking to the scammers again
In the Netherlands, in the 80s-00s at least, hanging up cleared your line, so picking it up again would get you a fresh dialtone even if the other side didn't hang up (and the other side would get a modified "busy" signal if you hung up on them).
Different telephone switches (the big ones in the central offices) have different behavior for clearing the line when one party hangs up. I've heard that some switches wouldn't clear the line for tens of seconds when a called party hung up, and that those switches were common in the UK.
I personally recall people ocassionally being able to hang up one phone before picking up a different phone if they wanted to take the call in a diffent location in the house. Although, I don't remember it ever being very effective, over time (in my corner of the US), it became never effective, so you'd just leave the first phone off the hook and have to hang it up later.
this was (is?) a "feature" that let you answer a call in say the front hall, then hang-up and continue the call in your office as long as the originating caller did not hang up the handset. The scam relies on actually playing a dial-tone and ringing after the victim "hangs up", which is how some people noticed it "sounded weird" and then called their bank on another line or cell phone.
Because when you hear the other side (pretend to) hang up, you need to not be fooled and also hang up on your side. If you don't hang up on your side, then on older phones you actually have no way of telling the call is still going.
On your smartphone, it's easier to not fall in this trap, as the screen is making it harder to type a phone number while the call is still going.
No, this isn't just confusion, this is a real behaviour difference between smartphones and (some) landlines. If someone calls you on a landline, depending on the exchange, hanging up your phone might not be enough to end the call; when you pick up again, you may still be connected to the caller.
It used to be the case that the party who made the call - being the party who is paying for it - was solely responsible for terminating it; then timeouts were introduced; and on many landline exchanges there is still a timeout before the call is actually terminated if the callee hangs up but the caller does not - although these days it is just a few seconds, that may still be long enough that you stay connected to the original caller if you hang up then immediately try to place a call.
However the scammers don't hang up they just play a fake dial tone so the victim dials the number thinking they're contacting their bank but they're actually just speaking to the scammers again