One time on April Fools Day my mom called her best friend. Her best friend was moving that day and her stuff had to be at her new house at a certain time in the day. Mom called and said something like "Uh, yeah, this is the truck company that's shipping your things to your house... um, your truck broke down, and it'll take about a week for us to get you a new one." Then she called back later and told her it was her. Funny stuff.
Then there are the classic Bart Simpson prank calls... heh. I doubt I should get into those.