virtually all carriers worldwide have allowed. FaceTime Audio uses about three megabytes of data for every five minutes of conversation, with FaceTime Video uses significantly more. Cellular talk time/minutes are not used after switching from a voice call to a FaceTime call. FaceTime calls can be placed from supported devices to any phone number or email address that is registered to the FaceTime service. A single email address can be registered to multiple devices and a call placed to that address rings all devices simultaneously. Standards The FaceTime protocol is based on numerous open industry standards but is not interoperable with non-Apple products. FaceTime's lack of interoperability makes customers dependent on Apple and unable to switch away from Apple products. Upon the launch of the iPhone 4, Jobs stated that Apple would immediately start working with standards bodies to make the FaceTime protocol an "open industry standard". While the protocols are open standards, Apple's FaceTime service requires a client-side certificate. FaceTime calls are protected by end-to-end encryption so that only the sender and receiver can access them. Apple cannot decrypt this data. Standards used include: H.264 and AAC-ELD – video and audio codecs respectively. SIP – IETF signaling protocol for VoIP. STUN, TURN, and ICE – IETF technologies for traversing firewalls and NAT. RTP and SRTP – IETF standards for delivering real-time and encrypted media streams for VoIP. Compared to most SIP implementations, Facetime adds techniques that enhance performance at the cost of breaking interoperability: port multiplexing, SDP minimization, and SDP compression. 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