Hi Jim Cook,
Try using the earphones with the microphone attachment and check if you can be heard OK on a call.
''If you can be heard'' then there is either a problem with the phone's internal microphone or perhaps the voice inlet hole on the bottom edge of the phone is blocked with lint or dust.
To check out whether the inlet hole might be the problem, use a strong light and a magnifying glass to see if it is blocked.
If it is (or even if you can't tell) use a vacuum cleaner, to try and clear the obstruction. Do not use the vacuum cleaner for too long at a time over the hole, use it in short bursts.
'''DO NOT''' use a pin or a probe to try and clear the hole. You might damage the microphone which is directly behind the hole.
''If you still cannot be heard'' when using the earphones with the microphone attachment, then there is a problem with the voice transmission circuitry in the phone.
You could try starting the phone in [https://www.greenbot.com/article/2923927/how-to-restart-your-android-phone-into-safe-mode.html|safe mode] to check if a downloaded app is the cause of the problem. If you can be heard OK this way then the trick is to find out which downloaded app is the culprit.
If it still doesn't work this way then there is a problem with the systemboard