Here's my approach:
nearly all telemarketing calls are long distance calls and my phone rings differently when it's a long distance call. So, if it rings differently and it's around dinner time I know it's telemarketing.
I pick up the phone and in a monotone voice I say "the number you have dialed is not in service, this is a recording. the number you have dialed is not in service, this is a recording...." then hang up.
The goal is to get my number off their list for good.
And if it was a real call, then the caller would call me right back, right? But the phone never rings back
Cheers,