Funny, we are on the TPS but get almost no telemarketing calls at home. I have added all my elderly relatives and they have been delighted with the results and find that for the odd calls just mentioning TPS gets rid of them.

At work we aren't on it and get several calls a day asking about changing telephone/electricity suppliers. The Indian guys are very easy to get rid of but the British sounding people were very difficult to get rid of (when I was polite to them) until I very quickly realised that it was easier to just tell them "I am hanging up now".

edit: On the technical question I had a slightly different problem a while ago (a spammer making abusive/annoying calls) and considered an automated screening system. Luckily they got bored, no thanks to Hants Police - I got their name/address from "a little birdie" but nothing became of it.

Gareth