Some people face hearing issues on iPhone during the call. As they can't hear anything from the other side during the call. When volume up key is pressed the sound appears very distorted. There are some potential fix for this problem.
You can try following things to fix up this issue:
- Press the "Volume up" button and do note that the ringer volume should appear on screen when you press the button, it should not appear on the headphones.
- Clean the speaker grill from outside. Do see if something is covering it up, small piece, dust o anything could easily cover it and could reduce the voice clarity.
- Try plugging in and out headphones, it could also help.
- Try to turn off the Bluetooth. Then try connecting it again.
- Go to Setting – General – Accessibility and make sure that the Phone Noise Cancelation is turned off.
- Try to use speaker phone during call and then turn it off.
- Reset the iPhone.
- Restore the iPhone.
If even trying these all your problem remains the same. It would be advised to check it with authorized service dealer.