when the phone registers, it reports it's ip address (which is internal)
the connection ip is also noted.
If 2 phones have the same connection ip, they are considered behind the same NAT and thus *can* local stream to each other.
If one of them is behind a second NAT (double NATTED), then that will not work. We assume that anything behind the NAT facing the internet is internally routable.
So is local streaming not working, ie no or one-way audio, or the calls have media going back to mbg?