In Cisco CallManager 4.x with CMM, if a conference call is established between the IP phones and the PSTN phone through the MGCP, there is only one-way audio from the PSTN to the IP phones and no audio between the IP phones

[TCC_2]

Core Issue

In Cisco CallManager 4.2(1)SR1b with the Media Gateway Control Protocol (MGCP), IP Phone A calls IP Phone B. The call succeeds. Then, IP Phone A calls a Public Switched Telephone Network (PSTN) phone, and this call succeeds. IP Phone A conferences in the PSTN phone with IP Phone B so that a three-way conference is established. The IP phones can hear the PSTN phone, but the PSTN phone cannot hear the IP phones. Also, the IP phones cannot hear each other.

Resolution

In order to resolve this issue, use the traceroute command in order to determine the route from the Communication Media Module (CMM) to the active IP phones. If messages such as these appear in the traceroute command output, remove the routing statement from the CMM, and replace it with IP route 0.0.0.0 0.0.0.0 :

ms_ac_rtp_udp_unreachable : loc_ip src_portdst_ip dst_port

Refer to the Configuring Enhanced Conferencing and Transcoding section of Configuring Enhanced Conferencing and Transcoding for Voice Gateway Routers for more information.