re: What's the best practical joke you've ever played?

Relatives got fancy new voice mail with the phone company, where your greeting and messages are on their server.

They weren't sure how to use it, so I set up their greeting for them: "Thank you for calling the Internal Revenue Service audit request line. Your request has been registered, and an audit team will contact you soon to schedule your first meeting. Please have all paperwork available."

Other relatives called us a couple weeks later saying they had not been able to get in touch with them. They said they called and called, but they "keep getting the damned IRS."