[Users] mediaproxy and registrar on two different server

Dear all, I'm working to separate on two different servers registrar and mediaproxy. Mediaproxy server (with both mediaproxy and ser proxy process) will be the front-end and all registers and calls from Intenet will be directed to this server and then routed to internal registrar. Now, I'm working with natted clients. I've started with only one server (with only one ser) and it works fine. When I've tried to split the architecture on two server I can't fine a good configuration. In particular, when a user with a public IP calls a natted user, the RTP doesn't use mediaproxy Anyone can help me? do you have any working configurations? thanks, Andrea