DTMF tones from T-20 not recognized

[chrisduncansb]

I have a T-20 running firmware 9.72.0.76. When I call a number on our PBX and try to select an option in the auto-attendant, the IVR does not respond to the key presses. I checked the PBX's logs and confirmed the PBX did not receive any DTMF tones.

I've tried every combination of DTMF settings on the phone, but none work.