Thanks for posting! Any automatic timing change in the unit is the result of the timing source being lost, or the T-1 going down in this case. Generally speaking, the T-1 clock for the Total Access 900 series units should synchronize to a carrier-facing T-1 or be set to internal if the unit has an Ethernet port facing the carrier. The Cisco CME should be set to synchronize its clock to the T-1 clock it receives from the Adtran unit. Typically this would be called "line" or "network" timing on the PBX.
You should first always verify that T-1 framing and coding settings match up. Then verify the timing settings above. If you take errors on the Adtran unit, we would typically plug the port into a loopback adapter to verify the port runs without errors to itself. If it takes errors when connected to the Cisco, you may want to attempt to adjust the LBO settings on that port. Below is an example.
interface t1 0/3
lbo long -15
It is rare, but occasionally I see this attenuation help clear up synchronization issues on the connected device. If you are unable to prevent the PBX from dropping the T-1 or sending an alarm of some kind, we would be glad to assist you further. However, you may need to place a T-1 test set between the two units to check for problems with voltage or frequency. In particular, I would check the frequency the Adtran unit is sending out and verify the PBX is sending back T-1 framing at that same frequency.
I actually tried this on Friday and got it to work.