Asterisk, IAX2, ZyXEL USG 200

Hi all, i have replaced an internal firewall pfSense (BSD firewall) with a ZyXEL USG 200.
We have an internet asterisk connected to a first firewall, connected to a frontend firewall, conncetd to internet


IP-PBX is mapped one-to-one–> and
Internal asterisk can communicate with another pbxs asterisk outside, and other pbxs can call us. (4569/udp is portforwarded)
This was true when twice firewall were pfSense, with ZyXEL when call outside, calls dropped with chanunavail, but INCOMING CALLS WORK perfctly.
Anyone have experiences with NAT and IAX2? i’ve read that iax not work with “all nat implemetations”, it’s true?
Can i use ZyXEL or i have to reconnect pfSense? :laughing:
i hope not…