I'll see all that and raise you the following for step four -
http://i88.photobucket.com/albums/k1...phoneprank.jpg
The paper put into the connection will prevent any signal so your colleague won't notice they have a call.
If they have those headsets with the foam covers a coin can be hidden, causing the sound to become muffled.
Finally, a small roll of paper shoved up the mouthpiece (small enough to be unnoticeable) will cause the customer to be unable to hear.
These three together can result in up to half an hour of confusion for your colleague.
Also see: replacing desktop icons with a wallpaper of a screenshot of the desktop. Delete the original icons and your colleague will be clicking away cursing his computer for not opening and software.
Step three can be further improved by using the 'scale' control on the monitor to make everything tiny.
Also you can have fun by setting up filters to forward messages back to anyone who sends them, have this filter copied about 50 times in Outlook on at least two machines and you can cause an email cascade that requires one machine be shut down. When I worked for the Revenue in Dundee myself and a friend generated over 500 emails to each other in 10 seconds, it crashed Outlook on mine and I had to reboot.
The very last tip is to lower your colleagues chair during lunch, push it under the desk, then raise it up so that it's stuck. Put vaseline on the height adjusting handle.
ALL the above kept me from becoming insanely bored at work.