These are instructions on how to enable Wi-Fi Calling on the iPhone.
Wi-Fi Calling lets you place a phone call in an area with little or no cellular coverage using your Internet connection.
Tap to launch Settings from your SpringBoard.
Choose Phone from the Settings menu.
Select Wi-Fi Calling from the Phone menu.
Tap to toggle the Wi-Fi Calling on This iPhone switch to the ON position then press the Enable to confirm your selection.
If Wi-Fi Calling is available, you'll see Wi-Fi after your carrier name in the status bar and your device will now make and receive calls over Wi-Fi.