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One way audio on 711a-law codec

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)?

 

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