But, if I don't hang up the received and the counterparty hangs up, then the line tone is the one of the "busy", not the one of the "free", and I'm unable to dial anyone until I physically hang up to reset the line. Or at least that's how it works back home. Is it different in UK ? I suppose it's something that less attentive people might fall for anyways.
In the scam in question, you (as the victim) - hang up the line. You then pickup the receiver to call your bank, via the number on your card.
The scammer, when they hear you hangup, plays a dial-tone down the line, so when you pickup the receiver, you have the impression that the line is clean.
You start to dial, they stop the 'dial tone', play a fake calling tone, and then 'answer' the call.
This would be incredibly malicious and difficult to detect even for the most skeptical of users.
Wow, this really is something if done right. I don't use a landline now, but if I remember, the caller needs to disconnect for the line to actually be disconnected. So even if the callee tries to disconnect by hanging up, the caller is still actually connected. If the callee picks up the receiver again and hears a dial tone, they'd be none the wiser. But I guess the scammer would also need to detect a key-press tone on the line and stop the fake dial tone, start the ring tone, etc.
That's not how it works with digital lines. Disconnect on any side breaks the whole circuit presuming they conform to even ancient PDH, much less SONET or SDH. Oh and this includes even more ancient ISDN. The trunk will immediately tear down the DS0 slot and circuit.
GSM and VoIP also do not allow this behavior without engaging call waiting on subscriber side.
This only happens with old fully analog connections. Not sure which country or public operator still has this kind of PSTN.
For the difference, peruse ITU-T G.175 and Q.522 standards. SE (switching element) will disconnect the routing on your side.
It took me a while to find the actual standard number.
Perhaps it worked on Strowger exchanges when the call is local? I definitely remember this being "a thing" back in the 1970s but I don't remember ever succeeding in reproducing it. Obviously there must have been some time out because otherwise you could DoS anyone's phone by just calling them then not hanging up.
In the UK, when exchanges moved to digital, they deliberately kept the old behaviour because some people relied on it (eg, hanging up their main phone, then walking to another room and picking up the call on an extension phone), with a timeout of a few minutes.
In 2014 they reduced the timeout to 10 seconds to make this fraud harder to pull off.
