I read a small note in a magazine about this.
Did you ever notice that when a telemarketer calls there is a delay after you answer? A few seconds of blank pause before the telemarketer greets you? Now you can use this to your advantage. Try this out the next time you hear the pause. Don't say anything keep quiet just listen. (You will be able to notice the background noises of the busy call center. You will also hear the telemarketer saying "hello... hello......HELLO..HELLO!! ARGH!" and then they hang up.
It's the tone of their voice and frustration that's absolutely hilarious.
I have tried this quite a few times now and I love it. Try it out.