Samsung Galaxy S9+
It's not possible to use Wi-Fi calling
Solution 1 of 4: 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 Phone.
Press the menu icon.
Press Settings.
Press the indicator next to "WiFi Calling" to turn the function on or off.
When Wi-Fi calling is active, the Wi-Fi calling icon is displayed.
Press the Home key to return to the home screen.