Easy call blocking
Why has no-one suggested a call blocking phone? We were plagued with nuisance calls until I installed a BT branded (I know, I know!) call blocking phone. Now everyone who isn't on my contacts list has to announce themselves before being passed through and, of course, the nuisance callers never do. If they did, I can refuse to accept the call and block any future attempts. Oh, and I use the Post Office for my telephone line not BT.
Simples!