[asterisk-users] SIP Codec negotiation

[Daniel Tryba]

On Thu, May 10, 2018 at 11:44:14AM -0700, Steve Edwards wrote:
> I receive an INVITE/SDP containing:
> 
> 	m=audio 11310 RTP/AVP 3 0 101
> 
> which I interpret as gsm, ulaw, rfc2833.
> 
> and I reply with an OK/SDP containing:
> 
> 	m=audio 15884 RTP/AVP 0 3 101
> 
> which I interpret as ulaw, gsm, rfc2833.
> 
> How can I tell which codec was actually used for the call?

AFAIK this is undetermined. The callee can send either ulaw or gsm,
unless the caller wants to narrow it down to 1 codec, see
https://tools.ietf.org/html/rfc4317#section-2.2

Most of the time the callee will pick the first (so in this case ulaw).
But there are media gateways out there that choose g711[au] above "more
complex" codecs regardless order in SDP. My prefer PSTN provider will
always prefer alaw if offered since that will prevent transcoding on
their side if the call goes to ISDN/POTS, but AMR if the call goes to
VoLTE.