I can use my wifi in airplane mode and make wifi calls. Have a Samsung note 10 plus. Had an LG G6 before this and that worked too. Used to do it all the time in Canada if I needed to call home to NY. After I turn airplane mode on it does shut wifi off but I can turn it back on but still be in airplane mode. Guess I am confused what you mean. I just turned airplane mode on right now and I am using it to post this over wifi. Maybe it depends on the phone or I don't understand what you are saying.
I would be happy to help with any questions you have. Calls made using WI-Fi Calling will show up on your bill. If you have airplane mode on, and using Wi-Fi or an app to make the call, then this will not show up on your bill. Please let us know if this helps.