I get about four phone calls a day, (presumably) from direct marketing types - it's not clear exactly who, because they're usually machine-dialled and there's nobody at the other end.
So: Telephone Preference Service. Let's see if it makes a difference. Takes 28 days to propagate, apparently.
[Update: Ironically, the confirmation email sent out by the TPS uses some sort of hash in the 'reply-to' address. As a result, it got caught by my junk email filter. D'oh! Incidentally - there's no check in place (that I can see) to ensure that the numbers I've registered are actually mine. I'm not registering anyone else for 'no junk phone calls' could ever be classed as a malicious act, but it's still a little weird that it'd be so easy, no?]
So far, my experience of the telephone preference service is surprisingly good. I have not had any junk calls from people I had not inadvertantly established some kind of business relationship with since signing up.