HUmmm... You know I really don't know, but I think, depending on the headset and the phones, that it may be possible to pair a headset with more than one phone at the same time, but not sure if during a call to phones #2 and #3, if it would connect to them, once it was already connected to Phone #1.
But the best way to find out would be to go to a store and ask for LIVE DEMO of their headsets or borrow one of the Reps headsets and try to pair it to two or more phones, and have them all recieve calls and try doing the Answer - Hold, Answer - Hold, Answer - Hold, routine.
Sounds like it should work, but ... the problem I'd see is how would the headset, know which call (phone) to go back to, after another phone had been answered, if it did work?