I just found this site and tested it out... And it works!
Basically it's a site that you pick out a prank message, enter a phone number, and in a few minutes or more, the site calls the phone and pranks them with the message you picked out.
Some of the prank messages on there would be the RickRoll, Peanut Butter Jelly Time, and 7 days (from the movie The Ring).
They claim to not share the numbers at all, and lots of people seem to have used it without any problems either, so I'm considering it safe.
I'm going to have some fun with this and my friends over the next few weeks... : D