I have been testing a scenario for a customer and have ran into this issue.
We have a remote site with a PBX that connects into a gateway via 4 FXS ports. The gateway is using MGCP signaling. Our intention is to use G729 between CM and the remote site. We are using CM version 6.1(3).
Both the remote site gateway and IP Phone are in the same Device Pool thus same Region. I have it set to do G729 intra and inter. However, when I place calls from either direction, the call connects but I hear no audio. The gateway is showing G729 when I do a 'show voice call status', 'show voice call summary', 'show voice port summary'.
Do I need a transcoder for this? If so I have tried that with the following configuration:
dsp services dspfarm
sccp local FastEthernet0/0
sccp ccm 172.19.72.220 identifier 2 priority 2 version 6.0
sccp ccm 172.26.24.237 identifier 1 priority 1 version 6.0
sccp ccm group 10
associate ccm 1 priority 1
associate ccm 2 priority 2
associate profile 10 register TRN001a6ca1f908
dspfarm profile 10 transcode
maximum sessions 6
associate application SCCP
I added the resource in CallManager as a Cisco IOS Enhanced Media Termination Point. I'm not certain if I was to make it that or a regular 'Cisco IOS Media Termination Point'.
I added the transcoder resource to an MRG and MRGL and assigned it to the MGCP Gateway in CallManager.
The voice gateway is a 2811 with PVDM2-16 ( 16-channel (G.711) Voice/Fax PVDMII DSP SIMM PVDM daughter card).
When I change the region to be G711 intra, the call works and I hear audio. When I switch it back to G729, the cal completes but I hear no audio. It definately sounds like a codec issue.
I don't completely understand DSPs very well or how the endpoints choose their transcoder.