Dear Sam, there were actually two versions that were made for UPS-AT, one with a mic selector switch on the right hand side and another that uses pushbuttons used to select the transmitter.
The one that uses the knob on the right hand side was our original PMA7000M-S. That phone must have the POtS interface (Plain Old Telephone System). Go to
PS Engineering :home and then click on Product Support and there you will find the installation manual.
If you have the one without that knob, that is an old version of our PMA7000B and that does have the capability to convert Com 3 into a full duplex interface. That you can find by going to
PS Engineering :audio select panels
Hope this helps.