Prank calls are funny. Anytime someone picks on the phone to a number that is unfamiliar, you put yourself at risk of being pranked. It could be someone you know and didn't save their number, it could be the wrong number, it could be a bill collector, or it could be someone on a soundboard making celebrity voices. Especially when they do it in a comical way.
Check out these videos of various celebrity voices prank calling people.
Peter Griffin Calls An Indian Restaurant
Joe Pesci calls a Black Guy
Arnold Schwarzenegger Prank Calls a Drunk Guy
Arnold calls the same drunk guy, he sounds drunker now
Mr. T Prank Call to a Tow Truck Company