Quast

For me, the humour in prank phone calls is in the reaction of the victim (most of the time, anyway)
When they know it's a joke and they play along it's not really as funny as if they think it's some heckling moron and get annoyed or outright angry.

One of my favourites on that site is the Jack Nicholson one (last time I checked there was only one) for that purpose.