I have just switched on wifi calling on 6s plus. Call in and out surprisingly works even if I switch on airplane mode. But sometimes when I try to call someone multiple times at very close interval, I got the caller message, number is busy now blah blah, have to switch off the wifi, then only it works.

While one of my friend's S7 edge if turn on airplane mode, it doesn't work and I can't even tell whether wifi calling is activated because on 6s plus, you would know as the status bar shows "U Mobile WiFi"

I think the others have answered you. Personally, my own spot at office has poor mobile reception, so sometimes making calls is almost impossible. But now with this wifi calling, regardless of calling in and out, the quality is crystal clear.

It is really good except when few occassions I was trying to call my client, 1st time was ok, 10 seconds later I made second call, it didn't go through and I got the auto-reply message,"the number is busy now bla bla" So I turned back my phone network back and like usual I move from my spot to another to make calls and it worked.