Core Issue
Analog telephone adaptors (ATAs) have a hardware limitation that only allows one port to establish calls with the use of the G729 codec. In the case of ATAs that use the Skinny Client Control Protocol (SCCP) in order to register with Cisco CallManager, this port is hardcoded by default to port 0, and therefore port 1 can only use G711 to establish calls.
Resolution
Since this is a hardware limitation for ATA, it is necessary to revert the Region Configuration on Cisco CallManager to use G711 codec.
Refer to Field Notice: Fax Passthrough Limitations on ATA 186 for more information.
