I have never come across a device that specifies RTCP ports in the SDP, so FreeSwitch is badly broken if it requires the peer to do so.
If it wants to receive SDP on a non-standard port, it is up to it to send the SDP attribute, not the peer.
It looks like we are talking about RFC 3605. Note that this says:
The RTCP attribute addresses a very specific problem, the
documentation of port numbers as they appear after address
translation by a port-mapping NAT. The RTCP attribute SHOULD NOT be
used for other applications.
The right solution for the Asterisk end in this case is to ensure that your RTP port number range is not remapped by your router. An ITSP really should not have NAT issues at their end, so should not be using the attribute. Any options that has should not constraints in certain cases certainly should not be a mandatory requirement .
I rather suspect that, although labelled as standards track, this is a dead end RFC, and the world has move to ICE for NAT traversal on SIP, although I don’t know how ICE deals with RTCP port mapping.