[Serusers] nathelper & rtpproxy

Hello, I have a question concerning the nathelper module and rtpproxy. Here is the network layout : Sipphone1 <----> Firewall with NAT <------> SER with nathelper <---> Sipphone2 (public IP) public IP/private IP + rtpproxy private IP (private IP) I understand that the regular way of using nathelper/rtpproxy is to put them on a dual interface computer (one internal interface and one external one). In my case it is not possible, the SER box is inside and has only one internal interface. I am puzzled at how I should configure SER and rtpproxy to allow sip calls from outside to inside. Currently the sip trafic is fine, ie I call from outside to inside, and my internal phone is ringing. The problem is with the rtp trafic. I can't find the way to make nathelper/rtpproxy change the rtp IP to be used for the call. I am using CVS version of ser/nathelper/rtpproxy. Whatever the config I try (examples, and mails from this list) the IP in the sip packet stays the one from Sipphone2 (where it should me the external interface ip from the firewall). Another way to formulate my problem is : how could I tell nathelper which IP it has to use to replace Sipphone1's ip for rtp trafic ? Thanks ! Guillaume