Conman calls a landline, starts conversation with "please call the bank back, im putting down the phone now" and ... just plays Dial tone. The trick is call ends only when the person who started it hangs up.
If they want to leave their line open "for a few days"... then good luck to them. :)
The idea was, maybe you dropped your phone and didn't mean to hang up and this gave you time to rectify your mistake without having to redial.
Scammers started exploiting this and the timeout was reduced to the order of a few seconds. Count to five after hanging up and you can just about guarantee that's a real dialtone.
implementing logic/timers with cogs is impractical.