http://www.tpsonline.org.uk/tps/

If you live in the UK register your phone number here and stop those annoying unsolicited phone calls. When you do get one, like I did last night you can get their details and then turn the tables on them by saying that you ARE going to report them. 9 times out of 10 they hang up, only a few are big enough to apologise

“From the site:

The Telephone Preference Service (TPS) is the UK’s only official ‘Do Not Call’ register for landlines and Mobile numbers. It allows people and businesses to opt out of unsolicited live sales and marketing calls.

It’s free and quick to register a telephone number.