[Serusers] NAT Problem with PAP2

Hi All, I'm testing the default of config of ser, and just adding some nathelper command to make it 2 UA's work, I have an x-lite and a PAP2 both are behind nat. X-lite is being detected by ser as behind NAT, but the PAP2 isn't, the ser thinks the PAP2 is registering with public IP. When I call from PAP2 to X-lite, i have no problem, because the c= IN on the SIP/200 OK reply is already public IP, so wehen PAP2 sends an ACK or BYE it sends it the c=IN of X-lite which is public IP. But when I call vice versa, that PAP send c=IN as private IP so when X-lite sends an ACK/BYE, it wil send it to the private IP. which obviously will not be able to be recieved by PAP2. How can I fix this? Another thing i don't get is that why PAP2 registers as not behind NAT, but when it replies to messages, it is sending the private IP on the c= IN. Anyone experienced this before? TIA Regards, Nhadie