How to turn Wi-Fi calling on ?

you go into settings -> cellular. If your carrier supports WiFi-calling, there should be an option displayed directly underneath your phone number. Also, you can do it in the settings of the phone app: settings -> apps -> phone. Official page: Make a call with Wi-Fi Calling - Apple Support

I have AT&T as a cellular service provider and I had to contact their support team to get WiFi calling turned on.