Today I have another amusing thing to do to your friends and enemies that I have to give credit to The Office for.  Basically it boils down to this, your co-worker walks away and leaves their phone on their desks.  You simply take their phone and pair it with your bluetooth headset.  Then when a phone call comes in you have the option of taking it for them.  Obviously you might want to be careful not to get into any awkward situations like answering a call from a collection agency :).