Richard Fuchs writes:
Possibly it's because of the non-standard RTCP
port in the answer.
Technically this is supported but the implementation is more of a hack
than anything else for the time being.
looks like the rtpc port number in 200 ok is not "non-standard"
anymore. from rfc 3605 dated october 2003:
The SIP messages use the encoding defined in SDP [RFC2327] to
describe the IP addresses and TCP or UDP ports used by the various
media. Audio and video are typically sent using RTP [RFC3550], which
requires two UDP ports, one for the media and one for the control
protocol (RTCP). SDP carries only one port number per media, and
states that "other ports used by the media application (such as the
RTCP port) should be derived algorithmically from the base media
port." RTCP port numbers were necessarily derived from the base
media port in older versions of RTP (such as [RFC1889]), but now that
this restriction has been lifted, there is a need to specify RTCP
ports explicitly in SDP.
-- juha