On the morning show of Z100 in New York they do a bit called Phone Taps. It’s basically prank phone calls, but they get callers to participate in trying to make their friends/relatives, freak out on the phone. Trust me, it’s funnier than it sounds in this description.
Check out their site at http://www.z100.com/pages/zoo/index.html
Then subscribe to http://www.z100.com/podcast/PhoneTaps.xml