Companies used to do certain checks like which one is having more strength wifi or network signal snd bssed on that your call will go. In my case i am having 50Mbps plan so ofcourse wifi is good. See screenshot below. Vol with wifi symbol.
Airtel now supports M40 wifi calling. I have M40 and after the latest update wifi calling automatically got activated. But unfortunately I have noticed that wifi calling icon frequently goes off. I have to restart my phone to again activate wifi calling.