Hi @tartaurus ,
Use a magnifying glass and a strong light and inspect the voice inlet hole found on the bottom edge of the phone to check if it is blocked with lint or other debris.
It is is (or even if you can’t tell), try using a vacuum cleaner to see if you can clear the obstruction.
DO NOT use a pin or probe to try and clear it as you may damage the microphone which is directly behind the inlet.
If this doesn’t resolve the problem, try using the earphones with the microphone attachment when making or receiving a call. If you can be heard OK when using this, then this proves that the voice transmission circuitry of the phone is OK and that there is a problem with the internal microphone. It is either faulty or has come loose from the systemboard.