I'm never going to hang up on a telemarketer again. Their companies value their time and hanging up simply allows them to call another number. I plan to keep them on the line for as long as possible from now on. Tell them you've got something on the stove that you want to remove and that you'll be back in just a minute. Tell them you're interested in what they've got to say. Put down the phone and walk away until they hang up. If they are going to waste my time at least I'll waste a bit of theirs as well. Besides, it's more fun knowing you're annoying some chump who isn't qualified for a real job but instead works in a menial job annoying people.
I just keep them on the phone as long as possible, occasionally insulting them, and then continuing on as if nothing happened. Finally I tell them I'm tired of the game, there was no chance they could ever get my credit card number, and I have successfully wasted their time. That usually results in a string of profanities which is even better!