I do heckle telemarketers. I don't know if anyone in here has seen the movie "1408" with John Cusack in it, but there is a scene that works perfect for a telemarketing situation.
Pretty much, John gets a call from the evil room and it says "We've killed your friends...every friend is now dead." in his hilariously menacing voice.
So one day, after a rash of calls from the same bloody people over and over, which began to drive me a tad overboard that week, I decided I'd just take up the phone this one last time and say in a very gritting smoker's cough voice, the quote above and drag out the last word until I lost my breath just to see how long he'd hold on.
TM: Hello may I speak to------
Me: We've killed your friends...
TM:...Wait, what? Excuse me?
Me: Every friend is now deeeeeeeeaaaaaaaaaaaaaaaaadddddd...
TM:.....
Me:.....
TM: *Click*
I was shocked, he made it until the end of the word dead. Never got a call from those people since.