Solved 3cx keeps sending PCMU to Grandstream GXW4104

[GeorgeTs]

Hello,

We have provisioned a Grandstream GXW4104 to our 3cx running Professional Annual 16.0.8.9.
The customer complains that they have no outgoing audio in outbound calls, while inbound calls work with 2 way audio.
I noticed that 3cx keeps sending PCMU to the GXW4104, instead of PCMA which has been set as the primary codec to negotiate in external calls under Settings/General.
I could not find any codec-related setting on the SIP Trunk / Gateway level.

Any suggestions to make 3cx suggest PCMA primarily when sending calls to the GXW4104?

Thank you,
George

 

[GeorgeTs]

Just to let you know that we just upgraded to v18 and the problem persists.
Now under Settings/Codecs I can see PCMA being top priority, but still under SIP Trunks / Grandstream GXW4104 there is no option to define outbound codecs priority.

The outcome is that still outbound calls have PCMU as priority thus this is the one used for RTP.
Any suggestion would be more than welcome. Thank you.

 

[ChrisC_3CX]

Now under Settings/Codecs I can see PCMA being top priority, but still under SIP Trunks / Grandstream GXW4104 there is no option to define outbound codecs priority.

The outcome is that still outbound calls have PCMU as priority thus this is the one used for RTP.
Any suggestion would be more than welcome. Thank you.

You cannot change this particular codec priority but, do bear in mind that since the call completes it means the gateway supports it, meaning it should not be the reason for the one way audio issue. If you do want to test this I guess the easiest way would be to remove the PCMU codec from the gateway's codec list. Since PCMA is the second codec on the list sent by the PBX, the gateway should negotiate on that. You cannot edit codecs for the gateway via the 3CX Management Console so you would have to connect to the gateway and make this change manually. I suggest you only do this as a test and to hopefully help narrow down the issue as we do not recommend nor support manually configuring IP Phones or gateways.

Before trying anything else, you might want to consider first confirming that the PBX is sending audio to the gateway by analyzing the RTP stream in the capture using a tool such as wireshark. You can check our academy "Basic Troubleshooting" course which explains how you could do this: https://www.3cx.com/3cxacademy/videos/advanced/basic-troubleshooting/

 

[GeorgeTs]

Thank you @ChrisC_3CX
Yes, we had tried to disable PCMU on the Grandstream and indeed the problem persists wit PCMA as well.
We now suspect the FXOs on the Grandstream or the FXS of the provider's CPE.
It is very strange that the issue is only spotted in outbound calls.

Thanks for your feedback.

 

[ChrisC_3CX]

We now suspect the FXOs on the Grandstream or the FXS of the provider's CPE.

This might indeed be the case though I do recommend performing the packet capture on the PBX first to confirm it, or at least confirm that the 3CX PBX is sending the audio correctly. Once you do, you'll at least be able to remove the PBX from the equation.

 

[GeorgeTs]

Yes, perhaps I forgot to mention that as well.
Both before and after the upgrade, both before disabling and re-enabling PCMU on the GXW4104, the PBX was sending the RTP stream correctly to the gateway, It is from the gateway and on that the issue takes place.

Thanks again for your support, unless you may have any further comments, please feel free to close this case.

The outcome on what concerns the PBX side is : We cannot define outbound codec priority on the 3cx side when it comes to a VoIP gateway.

Or in other words, the Codecs priority defined under Settings/Codecs (v18) deos not quite affect the provisioned gateways.

Thank you,
George

 

[ChrisC_3CX]

Thanks again for your support, unless you may have any further comments, please feel free to close this case.

I'm afraid there's not much to add at this point to be honest as we're pretty much completely blind as to what is happening so it really could be anything.

The outcome on what concerns the PBX side is : We cannot define outbound codec priority on the 3cx side when it comes to a VoIP gateway.

Or in other words, the Codecs priority defined under Settings/Codecs (v18) deos not quite affect the provisioned gateways.

This is true, it cannot be changed so what you found is indeed the case.