Hang-up cause 16, which is a normal hang-up. You are correct in assuming it is likely an network issue. You could do a wireshark and turn on SIP Debug from the Asterisk CLI to collect additional detail.
Unlikely, but you can try going to Settings > Asterisk SIP Settings > Under Nat Settings click on “Detect Network Settings”, Apply and try again.
Not sure if you have already read here, but maybe some clues.