The issue of Wi-Fi calling not working generally arises after an iOS update. While the latest iOS release has been the cause of happiness of many, it has brought a lot of shortcomings and bugs along with itself.
If you are unable to use WiFi Calling from your iPhone X, make sure that your device offers WI-Fi Calling and that your devices have the latest software. There are certain methods to enable WiFi calling. We will describe some of the methods to resolve this issue.
Reset network settings-
Use the airplane mode ;
Another way to enable WiFi calling from your device is by toggling the airplane mode.By turning the airplane mode on and off you will toggle any wireless connection on your device, including your cellular data.
Reboot iPhone for iOS 12 USERS ;
Performing a Factory Reset
This method is mentioned at the end because it will wipe out your iPhone X clean. Performing a factory reset will erase all the previous settings and you can use wifi calling once again. But it is advisable to take backup of your settings and data before performing the factory reset.
The process will take quite a few minutes. After few minutes you can either use the new settings for wifi calling or use the backup created before resetting the device.