If you are calling from a noisy environment, your phone filters out background noise, to make your voice sound clearer to the person at the other end of the line.
Noise cancellation is not available when using the loudspeaker or a headset.
For the best noise cancellation, hold the phone with the earpiece to your ear and the main microphone towards your mouth. Do not cover the secondary microphone next to the camera flash.
If you want to let the person at the other end of the line also hear the background noise, for example, the music when you're at a concert, you can temporarily deactivate noise cancellation.
Select and Device > Call , and activate or deactivate Noise cancellation.