My family has never bought anything over the phone, and yet we receive 2-3 calls per day urging us to do so. Since obviously the telemarketers are not being very efficient by calling us, and probably have gotten the hint by the time I've hung up, sometimes I think they're doing it just to annoy me. If they call your house and ask for the wrong name, and then correct themselves, or if they take more than 10 seconds to look up your name, it's a telemarketer. Answer all their questions with very loud, overdriven screaming. If the automaton waits for you to finish your response, place your phone by a loud speaker or jam down a touch tone button.
Calls made by actual humans are particularly annoying.
They are damn good at what they do. In fact, telemarketers are trained for weeks to be able to reproduce this tone of voice, for the purpose of making you feel sorry for them and sometimes actually listen to their sales pitch. Telling telemarketers that the person they called for has died is often an effective tactic. Telemarketing calls are not always a waste, though. An interesting and novel strategy is to try to sell them something in return.