Sprint's Wi-Fi calling began last February for the Samsung Galaxy S4 mini and the Samsung Galaxy Mega 6.3. Support for the feature was added last month to the Samsung Galaxy Note 3 via an update. T-Mobile has offered Wi-Fi calling since 2011.
The advantage to using Wi-Fi calling is that domestic calls and texts made and received while connected to Wi-Fi, will not count toward your monthly voice minutes or text cap. Starting today, registered developers will have access to iOS 8, and everyone else will be able to install it this fall.
Screenshot from WWDC reveals Wi-Fi calling for iOS 8