With WiFi On and Airplane mode On, I get a error message saying "airplane mode must be turned off to call".
Now I tested many times, turning WiFi off/on and airplane mode on/off and sometimes I can call over WiFi with airplane mode On, but most of the times not.
I have mullvad running over different countries and servers so I checked de server in use, but even on the same server I get different results so it seems a local GOS issue.
When I did get the error for turning airplane mode off and switched to another GOS user to try and call from there, I get the message something like "All lines are in use, close some lines to call", but there was no call to close. After a few times WiFi On/Off and Airplane On/Off, I could call again over WiFi with Airplane mode on.
Strange coincidence of things.