I used to say off the wall shit like, "I know it's you [enter girl's name], I told you I was going to blow my brains out!.. Why did you leave me for HIM!?!?" The trick is to always shout loud and pissed.
I learned about that over at ebaums, where some guy had sound effects set up and stuff. The telemarketer stayed on the phone freaking out for over 15 minutes.