I did something like that to a co-worker back when iwon.com started. He was using iwon for a home page. I got a bogus email address: iwon@winner.com, or something like that. I then emailed him saying he won one of... and then I listed that months prizes. I put a "call us at our closest iwon office" at the bottom of the email, and then I put the phone number of the woman sitting in the cubicle across from him. We each had a separate phone line, and I knew he wouldn't know hers.
Anyway he was all excited and he wanted to wait until he got home so his wife would be with him when he called. He was so gitty, I started to feel bad, but not bad enough to let him in on it. He finally broke down and called at work, a bunch of us including the woman across from him was in his cubicle when he called. As her phone rang he figured out it was her number. He hung up and called her a b!tch. I was the only one that knew of the joke, so she didn't understand what was going on. Before things got to crazy I told him it was me. He was pissed, but I think he was more disappointed. It's one of those jokes that seem funny, until you hear yourself tell it, then you realize it's not funny, your just an @ss for telling it.