Why can’t I enable Wi-Fi calling on my iPhone?

Wi-Fi calling allows users to make calls over a Wi-Fi connection when there is little or no cellular connection available. To enable Wi-Fi calling on your device you need an iPhone 5c or later running iOS 8.0 or later for T-Mobile and iOS 8.3 or later for Sprint. For AT&T you need an iPhone 6 or later running iOS 9.0 or later. Wi-Fi calling is only available on AT&T, Sprint and T-Mobile in the United States, Bell and Rogers in Canada and EE and Vodafone in the United Kingdom.