iPhone 12 Pro
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 and follow the steps below. You can also visit the EE web page for more information about Wi-Fi calling.
Press Settings.
Press Apps.
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.
Slide your finger upwards starting from the bottom of the screen to return to the home screen.