This was an issue with UK BT POTS lines some years ago. The line would 'hang' for a fair while after the recipient hung up before clearing down (disconnecting both sides).

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.