Try saying 'Just a moment; I'm sure you won't mind if I record this call' gets rid of the scam ones - they don't try again. No help, though, for the solar panel / you have won a holiday / cavity wall insulation type though.
Oh sorry, OmanYorkie, I didn't read the question properly (and I've moaned about people not doing that that myself in the past!). No, I haven't found a site that would help.
The following site won't/can't stop the calls but at least you may find out who it is calling and have a moan about it on site to other sufferers.
http://www.snozwangler.com/
was going to say BT telephone preference line but someone has beat me to it.
I still get them though ex directory. Auto dial is a pain in the back side as these are from US i think.