I am mostly a network guy, so please forgive my lack of understanding. When RTP packets leave a router from a loopback interface that involved in NAT, it appears that the source IP remains unchanged. We were attempting to solve a NAT/SIP issue where the "C" address was private in the SIP invite. For fun, we copied the public IP of the upstream device and configure it as a loopback on the router. That fixed the SIP "C" problem nicely but when RTP is also bound (media bound) to the same interface, we noticed that the source IP wasn't being changed by NAT but forwarded to the upstream router which then dropped the packets (probably RPF) producing one-way audio. We have tried some permutations of binding media to interfaces but with no luck. Does anyone know why RTP packets would ignore NAT? Is there a way to enforce NAT for those packets?
