Foretunately for me, I have call display. If I don't know the number coming in, I do not answer it. Period. If it is important, they will leave a message, and a message is never left. Another thing, long distance calls have a different ring tone, and most telemarketers a from Toronto.
I remember reading here a similar thread where when you pick up the phone (and know its not your mom or bud ;-) ). Push buttons randomly for 30 seconds and that this effs up the computer on the other side. Tried that a few times too. That was fun