No subject

your Asterisk server at all. Try doing a packet trace on the network
segment where the calling SIP agent is and see where it's trying to
send the ACK to. My guess would be your firewall is incorrectly
handling the SIP messages. Generally it's very bad news to use an ALG
or firewall to mangle SIP packets as they almost always get it wrong.

In your case there is a Record-Route header in the response so the ACK
request should be being sent to that address. Perhaps your firewall is
not correctly mangling that to allow the request to find its way back
to your Asterisk server.

Record-Route: <sip:216.82.224.202;lr;ftag=as3ed791f3>

Regards,

Greyman.