[Serusers] Redirect to unregistered client behind NAT

Hi all ! I have to redirect some calls to unregistered user (private IP) behind NAT This calls are forwarded to ext_IP:port and NAT redirect calls to this user. I'm not able to register this user on my server or set up external IP for him How can I achieve two way audio stream ? Do I have to use mangle module or rtpproxy should be enough ? Default configuation of ser for rtpproxy works only when this client is registered, but I can't register him. (it doesn't matter why ) When I make a call then only one way audio stream is hearable (from private to public) becasue public phone want get private address unregistered client. How does fix it ?? Thanks Andrzej