The uk Goverment is trying to stop unknown calls by forcing direct marketing companies to use CLI see here
You can respond by email answering the following 2 questions. (email address in link)
I did post this elsewhere and thought it may be of use here as well
I treat each one differently depending on what I'm doing.
1. hang up
2.let them waffle on whilst I put the phone next to the tv
3. tell them I died in the accident that wasn't my fault
4. tell them the accident was my fault and they need to speak to my solicitor
5. ignore the call and let it go to voicemail
In all cases, never ever sign up or agree to anything and always block the number.