Why doesn't the destination (PSTN) phone stop ringing when I have hung up my terminal?

When a user client hangs up, the client should send a CANCEL. This CANCEL must be routed so it reaches the dialled peer. As there still isn't a dialogi set up (and thus a known "path" from the Route headers), the CANCEL must be handled in your seri.cfg in the same way you handle INVITE, i.e. by doing (if any) request URIi manipulations and lookup("location");

The peer should then respond with an OK, and then later a 487 Request terminated.