I'm on the telephone preference service (mentioned above). If I get a call I usually tell them that and that they shouldn't be calling me. If they persist I then ask to speak to their supervisor. That usually does the trick and also ensures they don't try again.

The latest trick is computerised dialling. The computer gets a batch of numbers, dials them simultaneously and the first one to answer gets the call. The rest simply get a dead line. If you do a 1471, you get either a "network unable to transmit numbers" or a dummy 0870 number.

For non-UKers, 1471 gives you the last number that dialled you. 0870 is a prefix which charges nationally at local rates and is used by a lot of businesses for helplines and such like.