Wifi calling does need the cell towers initially to determine what's your roaming status, before you can use it. I have tried with different carriers (Indian and foreign) to keep the iPhone in airplane mode, and try to connect only to wifi in order to make a call. I've not been successful. I'm on the latest iOS. Region is a country that supports unrestricted wifi calling. It needs an initial connection to the tower, and then once wifi calling shows up in the status bar, I can turn on airplane mode and switch on wifi only. It'll work.