FaceTime uses Voice over IP
FaceTime Audio
A new audio-only version of FaceTime, named FaceTime Audio, was announced during the annual Apple Worldwide Developers Conference (WWDC) keynote speech on June 10, 2013, and released with iOS 7 on September 18, 2013. As an audio-only version of FaceTime, it effectively makes the protocol into a voice-over-Internet Protocol (VoIP), that competes with other mainstream providers in the field, including Skype (Microsoft).
Based on the same AAC-LD audio protocol, the service provides high-quality audio. The iOS 7 betas limited FaceTime Audio to calls placed on a Wi-Fi network (the same original limitation of the video version of FaceTime), but the final release has removed that restriction to allow it to work over 3G and LTE data connections, as is the case with most carriers and plans about FaceTime with video. Like the video version, FaceTime Audio is currently only available between Apple devices on iOS 7 and later. FaceTime streaming over cellular data is unavailable for the iPhone 4 and the iPad 2.
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