You shouldn't be getting any unsolicited sales calls if you've registered your phone number(s) with the Telephone Preference Service:
http://www.tpsonline.org.uk/ctps/number_type.h tml
It's often suggested that you should blow a whistle down the phone to annoy unwanted callers. Personally, I think that this is being a bit too nasty to the poor over-worked, under-paid person, working in a sweat-shop call centre, on the other end of the line.
Another frequently recommended tactic is to tell the caller that you're naked. This, apparently, will make them forget their script, and put the phone down.
I've not tried it, but I've often thought that the following response might work:
"Thank you so much for calling. I knew you would because the Lord told me so in a dream. Have you welcomed the Lord into your heart yet? I know He wants you to do so; it was in the message which He revealed to me. Please let me tell you more about Our Lord".
Chris