[Serusers] STUN and symmetric nat again

Hi! I've read about the above topic in the arcives and now I've got the same problem. User A is behind a symmetric NAT -> a new port is bound for every different (external ip, external port) pair. User B has a connection to the net without any NAT or firewall. User A wants to have a SIP+RTP session with user B. How? I guess basically what is needed is forming of a SIP message like "send the your reply to the same ip and same port from where you have received this" and user A can send this to user B. For the RTP session, a pretty similar thing would be needed like, A says to B: "I send you the RTP stream on your x port, send your rtp stream back to the same ip and port from where you receive mine". How to setup such a SIP packet? Or maybe there is a standard solution for this already? (STUN wont help in this case I guess, and TURN is not an option for me.) I've read about "symmetric SIP + symmetric RTP" maybe that is the same as I've described above? Thanks Istvan