One trick they use is to tell you they have taken control of your telephone line.

They use a very simple trick - they ask you to hang up and try to use the phone... when you hang up they mute their phone, so it sounds like your phone is dead, but it isn't - the incoming call on a land line holds the call until they hang up, unlike a mobile which disconnects irrespective of who placed the call.

A lot of people are taken in by it and panic, though. If you were just to hang up and leave the phone on the hook for about ten minutes, they'd get fed up and go away - and your phone would then work 'normally' again (well it was working normally already, though you didn't know it!).