T43U handset only works when headset button is green

[jaydub]

I searched and didn't find an answer, but then again I don't see the mayo in the fridge and my wife points to it and it magically appears

We have about 35 SIP T43U phones in our office. One phone's handset wouldn't pass audio either to earpiece or mic this morning. I replaced cord and handset and no change. I upgraded the firmware to 108..86.0.90 and no change. I reset to factory config then reconfigured...no change.

When I lift the handset then hit the "headset button" and it liights up green and the handset now works properly. The handset is acting like it is a headset for some reason. When I hang up the handset the call doesn't end and if I press the headset button again (green headset button goes out) it goes to speaker phone, then I can hangup to end the call.

I am the only one who can log into the phone and I haven't logged in before today for over a year.

Any idea what happened that the headset button needs pressed for the handset to work all of a sudden?

Thanks,
Jay