Try keeping a whistle by the phone for scam callers, I was pestered all day a couple of years ago, finally I dug out my old whistle from work and blew it down the receiver, there was a rustle of papers and silence from then on, bet the ears weres still ringing when the head set was thrown off.
There was a programme on channel 5 a couple of weeks ago about nuisance calls which was quite interesting, it said it had closed several call centres down in Wales and police were shown listening to tapes including the ones telling people they could get a free boiler replacement if they had an old one, I had several callers regarding this and would tell them the only old boiler in my house was me, they soon put the phone down.