Honestly, for faxing and t38, you should use g711 a/uLaw only or a high bit rate/high quality codec!

Fax is a 9.6kbps (or 14.4kbps) codec and g729a is 8kbps, a low bit rate codec. 

How do you expect to have it working using g729a which will compress fax data even more?

Check this article https://www.voip-info.org/asterisk-fax/  ... it is 20 years old, already, but still very accurate.

Atenciosamente / Kind Regards / Cordialement / Un saludo,

Sérgio Charrua

On Fri, Feb 14, 2025 at 6:48 PM Temilolu Fayomi via sr-users <sr-users@lists.kamailio.org> wrote:

Hello,

I am working with inbound calls on kamailio (specifically fax), and was wondering if there was a way to re-prioritize the codecs found in the sdp of an invite. I've looked at using the sdpops module, but haven't been able to re-arrange the priority of the codecs listed. I've tried removing codecs to help incoming faxes switch over to t38 per my upstream providers instructions (calls will do a t38 re-invite if the g729a is the highest priority). I am just looking for which direction to go from here or if I misunderstand the nature of faxing and kamailio.

Thank you in advance,
Temilolu Fayomi
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