Hi,
The problem with NAT (and btw firewalls aswell), unless you are static NAT'in outside-inside, the NAT relation between the UAC and OpenSER is strictly between the two.
The NAT router will se a packet coming to the right ip:port but not from the correct ip:port...
In short, if UAC1 -> P1 and UAC2-P2, an invite should follow:
UAC1->P1->P2->UAC2
br hw