I'm the same as most of you guys, I dont answer numbers I dont know. But if they're repeatedly calling... try one of these, it might make them stop... and you can have a little fun with it.
1. Let them start talking, and if they introduce themselves by their first name... act like you know them! Be all excited and say something like "Joe! Its been forever since I heard from you! How've ya been, man??"
2. I'm sure every one and their dog has seen/heard the clip some guy sent into a radio show where he dinked with a telemarketer by pretending he was the cops investigating his murder. Try that!
3. Let them talk, and if they ask for some one specific... reply in a really quiet, low toned voice and be as creepy as you can. Whisper (just loud enough that they can hear you) something like "I'm sorry... Carl cant come to the phone right now... you see, I've removed his legs...", then turn your head from the phone and shout, "I TOLD YOU NOT TO MOVE!" See how fast they hang up!
4. Start talking in jibberish. Or, if you're fluent in another language, start talking in that language. Really fast and REALLY angry. Or make up the language, but again... really fast, really angry.
5. Wait for them to start talking, put the phone as close to your mouth as you can with out swallowing it and scream at the top of your lungs for as loud as you can.
6. Get some one else in on it! Let the other person answer, put it on speaker phone (if you have it), then start shouting violently at the other person. Start at the back of the room, then stomp your way up to the phone while shouting. Knock the phone off the table if you want (just make sure it doesnt break!), and have the other person start screaming and acting hysterical... beg for forgiveness or some such. Have them scream bloody murder, then hang up telemarketer.
Have some fun at THEIR expense. They wanna bug us with pointless calls? Let em! Pay back is a riot.