RLyon,
Well, depending on the situation you have Group Listen which allows the user to let others hear the other side of the call through their speaker phone; not recommended in places where information is considered private; HEPA.
Then you have the standard dual headphones that someone else can plug in to.
The only other thing that I can think of is if they have a Call Recorder or if they Record Call then others can listen to the call afterwards.
Thanks,
TE