Learn about sound issue during calls.
1. Turn up volume to maximum, when you are calling;
2. Try another place with good signal to check if the issue remains;
3. If yes, try to change another carrier SIM card to check;
4. If you notice the same issue, try making a call to another contact to check;
5. If the issue still exists, switch the device to hands-free mode to check, if it is clear, you need to check if something went into the receiver such as dust particles;
6. Try to update your phone to the latest software version and reboot phone to check if it is OK;
7. If problem still persist, please go to local service center for help.