You might have noticed a small tiny black hole next to the camera on the back of the iPhone. That tiny hole is actually a secondary microphone. But this mic is not for voice recording, but for noise cancellation. When you speak on the phone with someone on a call, there may be background noises like those of cars and bikes, if you are on the road or the sounds of other people talking, etc. This mic is there to filter such noises out of the call, using the advanced audio system on the iPhone 5 and after models, and it gives you crystal clear calling experience. You can watch the video in which Phil Schiller explains how the audio system works in an iPhone 5.