[Serusers] using mediaproxy / nathelper depending on client and URI behind NAT

Hello all, I have done some test with the mediaproxy module. My config is based upon the sample code supplied with mediaproxy. This code sends all calls through the mediaproxy independent of if the user is NATed. I have made some changed to the ser.cfg by adding some client_nat_test options on INVITE. This works OK when the originating user is behind NAT -> mediaproxy is used. But if the originating user is public and calls a user behind NAT -> audio problem. I have searched the web + lists on how I can test if the uri is behind a NAT en thus use mediaproxy / Nathelper. I have also looked at config files for nathelper but it seems to have the same limitations. How should the ser.cfg be changed to enable mediaproxy / nathelper use based on URI ? Any ideas are welcome. thanks, Arne.