The other good one on a shared desk is to swap the handsets between two phones when they are back to back
Another one that's fun is to take the buttons out and put them back in calculator format, people don't notice the difference until they've had a few wrong numbers.
There was one occasion when we were sat around in the lab discussing the merits of a competitor's radio handset. One of the engineers had it in his hand, just the handset with no plug on the other end of the curly cord. All of a sudden there was a voice:
"Hello?...…...Hello, can anybody hear me?
Hello!"
To say that everyone was surprised that an unconnected handset could talk to us would be an understatement, but the guy holding it sheepishly replied anyway:
"Hello?"
"Hello, it's the operator here, can you put your phone back, I've got an incoming call and I can't make the phone ring because it's off the hook"
One of the guys had knocked the phone on the desk he was sitting on.