I’ve hooked up this headset according to this wiki. It sort of works, but not quite as expected. It seems to me if the headset light NOT green, and you pick up the handset, the handset should work as if there is no headset. I will get a dial-tone, but then the EHS switches to the headset. It only seems to work if I use speaker phone and then pick up the handset.
Right now, nothing is working on the handset, but I am guessing maybe the battery is dead. I enabled Headset mode on Endpoint Manager.