Is the phone in a protective case at all?
If it is try removing it from the protective case and check if you can be heard OK when in normal mode.
If it isn't in a protective case or even if it was and you still can't be heard, use a vacuum cleaner to clear any dust, lint etc that may be blocking the small pin size voice inlet hole that is located on the bottom edge of the phone.
Do not use a pin or probe to try and clear the voice inlet hole as you may damage the microphone that is located directly behind it.
If it still doesn't work OK then there may be a problem with the microphone that is used for calls etc when in normal mode i.e. not in speaker mode as that uses a different microphone. The phone would have pt be opened and tested to find out what is wrong.
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