There are three choices for Send DTMF: in-audio, via RTP(RFC2833) and via SIP info. It’s set to RTP(RFC2833).
On my PBX in a Flash all extensions are setup with rfc2833 for dtmf mode. My SIP trunks are also set up as dtmfmode=rfc2833.
In my sip_custom.conf there is no reference to rfc2833 - is that correct?