Apple have released Wi-Fi calling feature for iPhone X and other models (iPhone 8/7/6/5) a long time ago and many users find it really useful. The Wifi calling feature actually depends upon your carrier providers, if they offer it you can use it by going to settings->phone->Wifi calling. Like in USA almost all carrier provider (Verizon, At&T, Sprint and, T-mobile), in the UK (EE, Vodafone) and Canada (Bell, Rogers) carriers are giving Wi-Fi calling services to their users with iPhone.
But after upgrading to iOS 11.3 many iPhone users are finding that Wi-Fi calling is not workingon iPhone on iOS 11.3. Even on iPhone X and iPhone 8 users are unable to make calls using wifi after the upgrade.
The reasonyou are not able to make any wifi calling from your iPhone is that iOS 11.3 now prioritize on cellular calls. WiFi calling in iPhone only activates if your cellular signal strength is low. The WiFi calling will only turn on if you have less than 2 bars(almost) of cell signal. May be it is due to Apple's "Advanced Mobile Location" feature for sending your location to emergency services when calling from the iPhone. But this is not a bug or issue in iOS 11.3, infect a feature update from Apple in iOS 11.3.
Many iPhone users are not founding this change as user friendly as users sometimes prefer to choose on their own. The user may want to connect over wifi, even if the cell signal is strong.Also it may create issue for some overseas traveler/international travelers as they mostly rely upon WIFI calling during foreign visit.
Workaround to make wifi calling from iOS 11.3:
As mentioned in iOS 11.3 WiFi calling in iPhone only activates if your cellular signal strength is low, so you can turn on airplane mode and then turn Wi-Fi on. This will immediately turn on the wifi calling of iPhone X/8 and other models. It may not ideal because you can miss out on incoming texts (and phone calls, but they'd get charged for roaming anyway).