We used to put the on 'hold' and put them in a drawer. Just, "hey, theres someone at the door, can you wait a minute?" And then come back an hour later and see how long the cordless said the call had last.

Had one last 1.5h once.

These days, I just whistle a loud sustained C# into the phone that seems to pick up particularly well, and usually getts them to hang up cursing pretty quickly