Besides adding performance and user-interface improvements, iOS 8 beta 3 has activated WiFi-calling for iPhones on T-Mobile in the United States. The feature, which was announced in June, brings the ability for T-Mobile customers to make phone calls on iPhones over a WiFi connection instead of a standard 3G or LTE connection. According to people who have used the feature on iOS 8 beta 3, the calls sound far better over WiFi than over cellular. The Setting for enabling WiFi calling is found under the Phone preferences in the Settings iPhone app. As can be seen in the image above, users are required to enter their physical address for emergencies for WiFi calling. This is presumably to assist emergency services for people who call for help over WiFi. Apple also discusses the feature in the iOS 8 beta 3 release notes:
iPad Air 2
WiFi calling is one of several new Phone-related features in the pipeline for iOS 8. Apple has also completely redesigned the FaceTime application with a simpler look that separates video FaceTime calls from Audio calls. With Continuity and Handoff, users are now able to receive cellular phone calls via iOS Devices like iPads and iPod touches in addition to the Mac. Apple has also been working on support for Voice-over-LTE for future iPhone models. As more carriers support features like WiFi calling, Apple will likely continue building in support for iOS. Sprint is the other U.S. carrier that offers WiFi calling, but iOS does not appear to support that carrier as of yet.