Try using the earphones with the microphone attachment to make or receive a call.
If you cannot be heard when using the earphones with the microphone attachment, then there is a problem with the voice transmission circuitry in the phone
If you can be heard OK then there is a problem with either the internal microphone OR perhaps the voice inlet hole, found on the bottom edge of the phone is blocked with lint etc.
Use a strong light and a magnifying glass to inspect the hole to see if it is blocked at all.
If it appears too be blocked (or even if you can’t tell) DO NOT use a pin or probe to try and clear the obstruction, as you may damage the microphone which is directly behind the hole.
Try using a vacuum cleaner to try and suck out the debris.