The mux won't detect my internal ISDN Terminal Adapter.

The mux needs firmware revision 3.14 or greater to support the internal ISDN T/A . You can install the ISDN T/A in any MMH900 mux. With the older MMH900 PCB rev you will not get a ISDN LED on the front display. With the newer MMH900 PCB rev, there is a ISDN LED for the internal ISDN T/A module.