[Users] 200OK Contact header field.

Hello All, I have a problem with transactions rewrited to asterisk. In my configuration asterisk and openser are behind NAT on a Local Network.(Openser- 192.168.1.2, Asterisk 192.168.1.5) When a User who is behind different NAT(different LAN) sends an Invite which is then rewrited to Asterisk with (rewritehostport("192.168.1.5:5060");) everything works great till asterisk answers with 200OK. In this message the Contact header field is set to "192.168.1.5:5060". When user recieves this 200OK it sends ACK to IPaddress 192.168.1.5 (and since he is behind different NAT..the message goes nowhere). Since the Initial Invite in this transaction is "record_routed" by Openser I thought that the ACK should first go through Openser. Please corect me if Im wrong, and if possible, suggest some solution. Best Regards, Maciej Kowalski