For those of you rocking an iPhone 5c or later on UK carrier EE's network, today's iOS 8.3 update has brought support for the carrier's recently-launched Wi-Fi calling feature.
An EE community manager confirmed that the recently-released iOS 8.3 update includes support for the carrier's Wi-Fi calling feature on iPhone models ranging from the iPhone 5c up to the iPhone 6 and 6 Plus. To enable Wi-Fi calling, you simply navigate to the Wi-Fi Calling option in the Setting menu where you'll be presented with an option to turn the feature on. After enabling it, EE notes that it may take up to a day for Wi-Fi calling to be fully enabled on your device.
If you're unfamiliar with Wi-Fi calling, it enables you to overcome poor cellular reception by using a Wi-Fi connection to place a call instead. EE just launched its Wi-Fi calling a few days ago, but it is currently limited to just a few specific devices.
Source: EE Community Forums