Set up Wi-Fi Calling on an Apple iPhone 

To use Wi-Fi Calling, you need an Apple iPhone 5c or later on a supported carrier.

1. Open your phone settings. Settings gear icon shown here.
   
   
   
   
2. Tap Phone.
   
   
   
   
3. If Wi-Fi Calling is set to "Off", tap on the arrow next to it. If it is set to "On" your phone is already set up for Wi-Fi calling.
   
   
   
   
4. Tap the slider next to Wi-Fi Calling on This Phone.
   
   
   
   
5. Click Enable to finish setup.
   
   
    
   

Note that the look of your screen may be slightly different depending on your phone OS version or carrier.

Screenshots taken from an Apple iPhone 8 running iOS 12.0 on the Cricket network