Unfortunately for one of my co-workers I found the site Wakerupper.com.
The site is designed to let you schedule a wake up call. However, it works for prank calls as well. Unlike some other call-back services, you’re not required to register the phone you want to contact. Plus, you can type in text that the site converts to a voice message for the call.
The site is free and doesn’t require you to sign but don’t expect it to be around for long.