Plantronics M12 and Plantronics headset

Hooked up with
- telephone line cord from wall to desk phone
- handset cord into M12 port with handset picture
- M12 pigtail into desk phone
- headset with quick disconnect cord into form of M12

When I lift the handset, it works as usual.
When I press the handset button, the dial tone disappears from handset, cannot hear on headset.

I installed 2 fesh AA batteris, no change.

I opened the M12 side and moved the slide to all 6 numbered positions, no change.

The adjustable diode(?) is at the midpoint bewteen full + and full -.

What next?

Tim Crews
timothy.j.crews@lmco.com
(408) 756-9322

PS - sorry about indicating Phones & Communications/Conferencing Stations, but I had to select something. Can anyone help me troubleshoot and fix the M12 and headset?