The phone noise cancellation feature on iPhone devices, reduces the environment noises to improve calling quality. To turn on / turn off noise cancellation on iPhone 5, do as follows:
1. Open the Settings application from the home screen
2. Go to the General settings
3. Scroll down to the Accessibility settings and open it
4. Scroll down to the Hearing settings and to disable and turn off noise cancellation make sure the "Phone Noise Cancellation" option is toggled off.