Calls from IP phones towards the gateway or Public Switched Telephone Network (PSTN) work as expected.
Calls from the PSTN towards the IP phones may either fail completely or the user may experience one-way voice. Examination of trace files on Cisco CallManager reveal the gateway router is using a different IP address as the source of the call to what is actually configured on the Cisco CallManager.
The gateway router may have subinterfaces configured on the Fast Ethernet to allow traffic to be passed between different VLANs. The gateway router is using the IP address of a different configured subinterface on the Fast Ethernet port even though it may have originally registerd to the Cisco CallManager with a different IP address on another subinterface.
The call either fails completely or experiences one way voice if the source IP address the gateway uses is different to the source address Cisco CallManager is expecting.
The gateway can be forced to use a consistent IP address for all Media Gateway Control Protocol (MGCP) signaling and media traffic. In this example, the gateway router uses FastEthernet0/0.1 for the initial registration with the Cisco CallManager and for all subsequent MGCP control and media traffic:
mgcp bind control source-interface FastEthernet0/0.1
mgcp bind media source-interface FastEthernet0/0.1