One way audio on 711a-law codec

[Igor Aliyev]

Hello everyone.

Recently I have stumbled upon a peculiar problem. When the call happens between two phones according to the topology below sometimes a one way audio problem occurs (one call out of nearly 20-30). Here is the figure:

Cisco 8865 <===> CUCM 11.5 <==SIP Trunk==> VoiceGW Cisco2921 <==SIP Trunk==> CUCM 11.5 <===> Cisco phones

What is interesting, problem happens on Cisco 8865 and only when both phones for some reason switch to 711a-law. Usually they operate on 711u-law as both trunks are configured with MTP-termination on VoiceGW with primary codec as 711u-law. Moreover, transcoding is already configured on the router so there should be no problem with working on 711a-law.

So, here are several questions I am curious about:

-Why do phones might switch to 711a even though 711u is configured on trunk?

-Why does one way audio might happen with active transcoder on MTP?

-Where do I start to troubleshoot apart from capturing RTP traffic on phones (it would be problematic as problem occurs rarely)?

[Igor Aliyev · Accepted solution]

Turned out that the GW didn't have route to one of the phones subnet and when it was used as an MTP no audio occured on one side of the call.

[lisaedward]

Experiencing one-way audio on the 711a-law codec can be attributed to various factors. This codec, commonly used for VoIP communication, may face issues such as network disruptions, incompatible configurations, or device-specific problems. Troubleshooting steps could involve checking network connections, verifying codec compatibility, and inspecting device settings. If the issue persists, seeking technical support or consulting the platform's documentation may provide further insights for resolution.