Why don't you talk to Mike Sandman (
www.sandman.com)? He modifies just about every type of handset thre is with push to talk, push to mute, amplified handsets, noise cancelling handsets, etc.
I would think it would be possible to modify the handset so that a supervisor could listen into a conversation using the two receiver wires going into a Melco or Viking Silent Observing unit instead of a real dial tone.
If he could do that, a supervisor would dial a code to listen to the voice coming out of the particular receiver and be able to speak.
Modifying headsets is over my head, but if it can be done, Mike Sandman would be able to do it.
Is it worth enough money to modify a bunch of handsets and do a silent observing system (which I have a bunch of by the way.)
One other way would cost some cell minutes but would work perfectly.
The supervisor barges into the line via Toshiba or silent observation unit and listens. If he wants to feed info, he calls a cell phone (listening via a headset in his other ear) set to silent ringing and talks to the salesperson through an earphone (corded or bluetooth) resting in the salesperson's ear.
Customer hears nothing, sales person has cell phone earpiece in one ear, headset or receiver in other ear. Supervisor listens in with his phone and calls the salesperson's cell phone (which lights up, no ringing) and he/she answers listen to the supervisors pitch in one ear and the customer in the other ear. Just buy cell phones with lots of minutes. THIS WILL WORK.