iPhone 6s
It's not possible to use Wi-Fi calling
Solution 1 of 5: Wi-Fi calling is turned off
When Wi-Fi calling is turned on, you can make and answer calls using a Wi-Fi network when you're in an area with little or no network coverage. To turn on Wi-Fi calling, you need to establish a connection to a Wi-Fi network, select 4G (LTE) as network mode and follow the steps below. You can also visit the EE web page for more information about Wi-Fi calling.
Press Settings.
Press Phone.
Press Wi-Fi Calling.
Press the indicator next to "Wi-Fi Calling on This iPhone" to turn the function on or off.
If you turn on the function, press Enable.
Press the Home key to return to the home screen.