[Serusers] SER -> PSTN Gateway+NAT: BYE handling problem

Hi all, I'm running the following schema: UA ( possibly natted ) -> SER -> PSTN Gateway I have a problem with UA belonging to a particular network with private address not RFC1918 compliant ( class 1.x.x.x ), SER and PSTN Gateway have pubblic address. The problem is that, after a succesfull call, if the PSTN gateway send a BYE to SER, then SER forward BYE to the private address of UA instead of pubblic one. I don't understand which is the section that handle BYE messages and how can I solve this problem: anyone may help ? Second, another question: with this particular network I had problem with INVITE too, because SER was sending "proxy authorization request" to the wrong TCP port. To solve this, I've moved the nat handling ( with force_rport ) before the proxy_authorize block and it's working, but why this is not necessary on standard RFC1918 compliant natted address ? Many thanks for any explanation Fabio