[Serusers] MediaProxy multiple interface probelms

Hi, I'm having a problem with mediaproxy. I'm trying to proxy through from the internet to a SIP switch which is on a private network. I have a machine that has connectivity to both the internet and the private network, but as the request comes in on the public interface, if you issue a use_media_proxy(); it changes the "c=IN IP4"  header from the initial INVITE from the SIP client to the external address of SER instead of the internal one, which means that the SIP switch's RTP packets can never reach the mediaproxy, as the switch doesn't have a valid route to the internet. I can establish a call, but obviously the SIP sends it's RTP packets to the external address so they never get there. Also, I noticed that the SDP that the client receives also contains the private addresses of the cirpack switch. Could someone give me some tips on what the best way to "bridge" two separate networks with mediaproxy is? JPF