I got so pee'd off with all the calls from various sources (not just the computer one) that I have Caller ID enabled and changed two of the phones in the house (being on the TPS list doesn't seem to stop the marketing calls any more).
This works well, except when

is checking for the caller number - if she doesn't recognise it, she wont answer the phone. I have missed several calls from friends since setting this up!
However, as many of the scam type calls that JT mentions seem to come from abroad, any International Call is held now. I'm beginning to wonder how many numbers I can add to the blocked number list.
It doesn't stop the phone ringing, but does give you a very good chance of never having to talk to these thieves etc. again.