As seen on Apple’s iPhone tech specs
webpage, both the iPhone 6 and its larger iPhone 6 Plus sibling leverage next-generation H.265 technology, also known as High Efficiency Video Coding (HEVC), for encoding and decoding FaceTime video calls over cellular. The phones also support the older H.264 standard first championed by Apple with legacy devices like the Apple TV and third-generation iPad.
According to Matthew Fleming, an expert in the field of signals processing who spotted the new iPhone 6 feature, H.265 promises to deliver video quality identical to H.264 AVC at only half the bit rate. This translates into a massive overhead reduction much needed in constrained data systems like cellular networks.