I have an Asterisk server with a public IP address (without using NAT). Additionally, I have two IP phones behind different NATs, each with a different mapped public address. Both phones can register on my Asterisk server, but when one of the IP phones calls the other, I observe a loop of received INVITE packets on Wireshark on the Asterisk server without seeing the TRYING and RINGING packets. As a result, the SIP session never establishes.