The one sure way to stop most telemarketers is to ratchet the talk up to 100 right off the bat. I have less than 0 patience for that shit. Rarely ever do I get a 2nd call from any of them anymore and it's worth it.
For the record: I rarely answer an unknown number unless I'm waiting for a call... which infuriates me even more as those are the cases in which they're really wasting my time.