Hi,
Check that the voice inlet hole on the bottom edge of the phone is not blocked with lint etc.
''Do not'' use a pin or probe to try and clear it as you may damage the microphone that is located behind the hole.
Use a vacuum cleaner to try and suck out any debris that may be blocking the voice inlet hole
If still no good, try starting the phone in [https://www.getdroidtips.com/boot-mate-20-lite-safe-mode/|safe mode] and check if it works OK. If it does then a ''downloaded'' app is the cause of the problem The trick is to find out which one. A ''downloaded'' app is one that you downloaded and installed on the phone and wasn’t already pre-installed when you first got it.