Why is this hole in the smartphone?

Why is this hole in the smartphone?
Your smartphone has a hole, must have noticed? But have ever thought, why this hole is kept? If you know it will be good.

This is not a button. Even if you hear it, it will be a surprise, not even a part of the camera. It's a microphone. But what does the general microphone mean, it's not exactly the same. It's a totally different type of microphone.

Its name is Noise Cancellation Microphone. It is called "rear microphone". Having a name is definitely an idea about the job. The microphone is obviously because of what the phone is listening to.**