We just discovered that if a Customer has the USB cord from the headset base going into the phone AND they also have the RJ9 connector cord going from the headset base to the phone, that will “disable” the headset jack.
This means no audio will come through even if the headset button is lit. All we need to do is unplug the USB cord from the phone.
This probably won’t happen very often since the set up was just wrong, but if it does. NOW WE KNOW :)