I finally got it to work with the help of support@gl-inet.com . First I installed WinSCP and logged into my router on WinSCP, found the file /etc/modules.d/nf-nathelper-extra and added a #to the beginning two lines:
- #nf_conntrack_sip
- #nf_nat_sip
Saved, logged out, rebooted router, and tried calling on my phone in airplane mode and I still get the massage “”turn off airplane mode to make this call” . Then support had me install putty so I could write these lines into the router code:
uci set firewall.@defaults[0].flow_offloading='0’
uci set firewall.@defaults[0].flow_offloading_hw='0’
uci commit firewall
/etc/init.d/firewall restart
See this link for instructions on using putty SSH to the Router - GL.iNet Router Docs 3 (gl-inet.com)
After this wifi calling worked on my GL-SFT1200 opal router.