I can't remember the last time I had a cold call, but I answer every call I receive.

I know the 'amusing' thing for people to do is to piss around with the person on the other end of the phone, but if it's a cold call I just politely tell them I'm not interested and put the phone down.
Most of the time the person is just a kid on the other end of the line working for next to no money (probably on commission) in a job that is making their life hell. They don't need me adding to that by stringing them along like an arsehole, they're just trying to get by.