NAT can interfere with SIP and RTP by changing the ports on the way through. To counter we enable NAT traversal by default therefore communicating directly to the port that sent us the original RTP traffic (instead of sending back to the RTP port in the SIP message). For SIP registrations therefore you should normally disregard any IP address and ports shown in the SIP message as in reality they are rarely the ones used.
Firewall Behind NAT
You can generally disable firewall rules if behind NAT as they they shouldn’t be required. However, if port forwarding [SIP and RTP] selecting an appropriate RTP range can be complicated. In this scenario you may need to disable NAT.