That just sounds like the rtpproxy is not being engaged, i.e. that the rtpproxy_manage()
call is failing. When that happens, the SDP from .2 will be passed through unaltered.
The Kamailio log should give you some idea of why the rtpproxy invocation has failed.
RTPProxy is certainly not limited to RFC 1918 addresses or any particular network
characteristics.
-- Alex