To "prove" that ALG is off, target a host on a public IP with your SIP client, and run a packet capture there. In the "Contact: <sip:someuser@192.168.34.132>;expires=3600" line of the registration ...
I hope I can explain this correctly, sorry if I don't. Some 'assumption' on my part is included. I assign static IP addresses to the clients (in the client, not the Router), always have done so. Two ...