No one can hear me
When I answer my phone, no one can hear me and NO the MUTE feature isn't on.
Try connecting the earphones with the microphone attachment to the phone and then make or answer a call.
If you cannot be heard using this method there is a problem with the voice transmission circuitry in the phone, which would involve a repair/replacement of the systemboard.
If you can be heard using this method it proves that the voice transmission circuitry is OK but there is a problem with the microphone in the phone.
It may be that the microphone is faulty, has become disconnected from the systemboard or the microphone inlet has become obstructed. The first two would involve the phone having to be opened and initially be visually inspected.
You can check the third possibility by using a magnifying glass and a strong light to check to see if there is anything clogging up the small pinhole on the bottom edge/side of the phone. Behind this hole is the microphone. If it does look like there is some lint, dust etc do not stick a pin or anything else into the hole to try and clear the obstruction, you will damage the microphone. Try using a vacuum cleaner to suck out any obstruction. Do not apply the vacuum cleaner for long periods to clear the hole, try short bursts.