Nothing is more distracting than background noise when you are trying to carry on a conversation. In order to help minimize those distractions, iPhone’s that run iOS 7 or later has a noise cancellation feature you can use to improve the sound quality of calls.
In order to improve the sound quality of iPhone call, launch the Setting app from the Home screen and tap on General. From that point, tap on Accessibility and scroll down until you see the Hearing section and enable Phone Noise Cancellation.