Making progress here and hoping it would help others…
When making the SDP offer, if I change RTP/AVP to RTP/AVPF, FreeSwitch sends the SSRC: m=audio {mport} RTP/AVPF {payloadType}
v=0
o=FreeSWITCH 1587659473 1587659474 IN IP4 xxxx
s=FreeSWITCH
c=IN IP4 xxxx
t=0 0
a=msid-semantic: WMS klNy6C0zPnF1GHimljgWwJ6uh2YZVTN5
m=audio 23548 RTP/AVPF 100 101
a=rtpmap:100 opus/48000/2
a=fmtp:100 useinbandfec=1; stereo=1
a=rtpmap:101 telephone-event/8000
a=ptime:20
a=rtcp-mux
a=rtcp:23548 IN IP4 xxxx
a=ice-ufrag:Jos5Xoq5hfU3YyYU
a=ice-pwd:PjLzv2ZUQgPaGXGgbYbZdsUl
a=candidate:9347119714 1 udp 659136 xxxx 23548 typ host generation 0
a=end-of-candidates
a=ssrc:1460839061 cname:5ru1UlrAzXr5uzhG
a=ssrc:1460839061 msid:klNy6C0zPnF1GHimljgWwJ6uh2YZVTN5 a0
a=ssrc:1460839061 mslabel:klNy6C0zPnF1GHimljgWwJ6uh2YZVTN5
a=ssrc:1460839061 label:klNy6C0zPnF1GHimljgWwJ6uh2YZVTN5a0
Will continue after dinner