With the arrival of iOS 8.3, Sprint is rolling out Wi-Fi calling to iPhone customers using an iPhone 5c or later.
Sprint announced that customers running the recently-released iOS 8.3 update are now able to use the carrier's Wi-Fi calling feature. The same capabilities came to customers on U.K. carrier EE earlier today. A new Wi-Fi Calling option will appear in the Settings app on iOS after you update to iOS 8.3 to let you switch the feature on.
Wi-Fi calling allows you to make calls over your Wi-Fi network instead of a cellular connection. Sprint's Wi-Fi calling on the iPhone is supported on devices from the iPhone 5c to the iPhone 6 and iPhone 6 Plus.