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.