We set the phones up in a loop one April Fool's Day - somehow enough of us managed to get in before the receptionist (who we were playing the joke on) to get it done.
The really daft thing is, the phones couldn't actually forward to one another (or if they could, no-one knew how), but they could show messages such as "not at desk, please call nnnn". So she called a number, saw the message, called the next person and so on.
We did take care to make sure it didn't happen to any real calls - well worth the money spent in the phone box over the road (this was before mobile phones were an option!)
I've also "accidentally" disconnected a colleague's call with their manager before now - well, there's no point in looking after the phone system if you can't help the people you work with occasionally
(somehow doing IT support meant I was the ideal person to take on the phones, too...)