I add t140 text support in my sip.conf:
[general]
allow=t140
allow=t140red
textsupport=yes
videosupport=yes
allow=h263
The SDP offer from asterisk looks like:
media=text 16452 RTP/AVP 106
a=rtpmap:106 T140/1000
a=sendrecv
There is no payload for RED for t140 redundancy support. My asterisk version is 14.4.0.