I feel your pain
I get a frankly ridiculous amount of nuisance calls, some on the landline but most on my mobile, which I attribute to having had to have put that number down in past years when entering competitions (etc.) and having my details sold on.
To my mind these cold callers (typically outsourced to India and the Phillipines, in my experience) are the bastard scumbag vermin bastard vermin scum of the earth - I know, they know, they know I know, I know they know, that they're just reading from a prepared script and calling at random from what is no doubt a very long list of random numbers. But seriously; have sex and travel. No, I've never been involved in a car accident. No, I've never taken out PPI. No, my computer is fine. I know I haven't ever had any issue with these things and here's the thing,
so do you.
In my experience they can't be stopped before but only after the fact, that's to say by blocking each number on your mobile once you've received a call. Don't answer a suspicious call; wait for the call to ring off then Google the number and, if necessary, block it. Websites such as whocallsme.com (there are many other similar ones available) are invaluable.