I have two FreePBX 16 installations that can ping, SSH to each other but I never get an ACK after a ping and poke when I debug the IAX2 trunk. I Have 7 other systems that can establish a IAX2 registration and show up as a peer but not this unique connection. I see the ping an d poke but no ACK for this connection,
After doing some packet captures I found one of the customers firewall is having a problem passing the IAX2 ACK over their VPN. the issue was a UDP SYN security system that did not like the PING, POKE ACK proocess for IAX2.
The Cisco Firepower by default identifies IAX2 as “anomalous” packets in the SNORT IPS engine and blocks it. You will have to set trunk=no in your peer details and a trunk mtu=1280 or lower for the firewall not to block it.