[Serusers] help! lost invites when using outbound proxy

Hi everyone! I have set up a ser as an outbound proxy with nathelper+rtpproxy. This is my set up IPphone1 --> nat router --> outbound proxy(OP) --> softswitch(SS) IPphone2 --> with the OP and SS in public. The IPphones are behind the same NAT and both configured to the same OP. I am only testing against this softswitch from some vendor so i don't have any control over it. And I used the sample nathelper.cfg from the CVS to get started. Registration went smoothly. The problem starts when I try to call one phone using the other. Nothing happens until I recieve a 408 request time out. I did dumps on the OP and NAT router and what happens is the that the SS sends the INVITE to the NAT router directly instead of sending it to the OP. I tried reading the RFC but since im still new with sip and ser i am still at lost. Maybe someone here can help enlighten me. On sending INVITES, where in the SIP messages does the SS knows where to send the INVITE? Is it suppose to send back on what's in the Contact Header in registration? Would a Record-Route help solve the problem? Here's a rough description on what happens on the INVITES on phone1 calling phone2: IPphone1 --> NAT --> OP --> SS Adds: Via Changes Contact with source ip IPphone1 <-- NAT <-- OP <-- SS 407 407 407 407 Removes Via IPphone1 --> NAT --> OP --> SS Invite with Adds Via Authorization Changes Contact with source ip (lost) NAT <------------------------------ SS sees 3 Vias Thanks in Advance for any help or ideas. Regards, CGR