Get a BT nuisance call blocker cordless phone system.
BT7610 is excellent and blocks most unwanted calls.
You can block international, Payphone, withheld or number specific.
Since getting one my nuisance call problem is virtually gone.
Phone spammers can now spoof the cli data so this problem is never going to stop, but you can reduce the amount you get.
Robert Dyas usually have them on special offer.