[Sparkologist]

When you get a cold caller, don't hang up. Put the handset down but don't put it back on the cradle or switch off. Whilst your phone is still connected to that call, the cold caller cannot ring out on their phone until you hang up. You will hear all sorts of banshee like screams and alarms coming from your phone, telling you to hang up, but it's a good feeling when you know that the nuisance caller is getting a taste of their own medicine, and deny them the opportunity to do their job and earn some commision.

I learned of this technique from my cousin who works for BT.