I have heard somewhere that wifi calling uses a small amount of data to set up a call before going straight wifi. If this is correct, then you couldn't make a wifi calling call in Airplane Mode because data would be off in Airplane Mode. AT&T allows you to make and receive wifi calling calls while overseas to/from US numbers. If the answer to the data question is yes, then these calls wouldn't work while in Airplane Mode because you want to eliminate data charges. Secondly, if you can take advantage of wifi calling overseas, would you have to dial +1 then the US number or just the area code and number?
Don't really recall anything about small amount of data being used to set up a call, at least not data that wouldn't be used from WiFi that would be used for the WiFi call itself. Dialing from outside of US works basically just like within US and works just fine with Airplane Mode enabled and WiFi turned on separately.
I have used wifi calling from an overseas location, airplane mode on and wifi on. If something happens to the wifi connection it may default to cellular that is why I turned off cellular.