[Serusers] How to change connection information in SDP

Hi, Is it possible to change the connection information (c tag) in the SDP? My media gateway (SIP->PSTN) has local IP addresses. I made a NAT translation on the router to make it accesible to the public network. Softphones in the public network were to able to successfully registered on the SER proxy server. The problem is when i start to make a call. The OK message that is sent by the media gateway has an SDP whose connection information has a local IP since it doesnt know anything about the translation on the router. Although the signaling are all correct, the RTP ports are not able to successfully connect and as a result the parties are not able to hear each other. So I need to change the local IP on the SDP with the mapped public IP on the router before passing it on to the caller. Thanks, Ian