I have no problem of multiple phones behind the same NAT, registering with an asterisk server outside NAT (Pubic Address). Phones can call PSTN via Asterisk, or other phones behind other NATs with no problem.
But these phones can’t speak to each other. They can ring each other (sip working). But when you pickup the phone, there are no audio (RTP not working). I can see the RTP packets are dropped on the NAT device with icmp port unreachable error. The source & destination RTP ports are correct. The NAT device can’t forward these packets to the phone on its inside interface.
Does this surpose to work? i.e. phone-to-phone behind the same NAT device, with asterisk service outside NAT.