Transcoding G.722

[mike748]

Hello,

I have an issue with external calls when checking box "PBX Delivers Audio" at SIP Trunk level. It looks like the phone is establishing the call using codec G.722 (on the phone Yealink T46S phone I can see the "HD" label) but the leg between the PBX and the SIP Trunk is using G.711a. The quality becomes then very bad, most likely due to transcoding.

I am using the following setup of codes (priorities):
- Extensions: G.722, PCMA
- SIP Trunk: G.711a

I want to keep the G.722 codec for internal calls but I would like the external calls to be established with codec G.711a.

Is there a way to force 3CX to use the same codec on both legs to avoid transcoding?

Thanks for your help.

 

[eddv123]

You can set preferred codecs on extension and on the trunk but if the phone and trunk do not support the codec it will use the next supported codec in the list (based from top to bottom).

FYI this is also part of the job of a professional SBC (Audiocodes, Sonus, Patton etc) - codec negotiation between PBX and trunk.

 

[YiannisH_3CX]

Hello @mike748

As mentioned by @eddv123 the codec that will be used is determined by the device that offers the call. So if the provider offers G711 as its first codec and the PBX supports it then G711 will be used between the provider and PBX. Then the PBX will send an Invite to the extension. If the first codec of the PBX is G711 and the extension supports it, then G711 will also be used between the PBX and the extension. In the upcoming SP6 you will be able to select the codec order of the PBX