I have the same crap also. Daily I get a call for energy, phones, internet, insurance, PPI, survey's, you name it I get it. Half the time I tell them to F'off, but on some occasions I like to wind them up.
I asked one company how they managed to get my number, they said via random dialing. A computer dials numbers in sequence until it receives a ringing phone, at which point the "caller" is then connected with the call, which is why sometimes you get that blank gap before they speak.
It's the world of marketing and harassment...... totally uncalled for and a bloody nuisance! I now block all of these nuisance callers if they display a number.