iOS 8: Wi-Fi Calls or iPhone Cellular Calls
iOS 8 brings two features that are mutually exclusive: Wi-Fi Calls and iPhone Cellular Calls.
Wi-Fi calling will be turned off to allow other devices to make and receive calls through your iPhone.
Wi-Fi Calls is currently enabled on carriers that support the feature, such as T-Mobile USA.
iOS 8 also enables Cellular Calls handoff to OS X Yosemite and other iOS 8 devices.