Try using the earphones with the microphone attachment and make or receive a call.
If you still cannot be heard properly then there is a problem with the voice transmission circuit in the phone.
If this is the case, verify the purchase date and then consult the warranty statement that came with the phone's documentation as to what to do to make a claim for a warranty replacement.
If you can now be heard properly then there is a problem with the phone's microphone. It may be faulty, loose on the systemboard or the voice "inlet hole" on the bottom of the phone may have become blocked with lint etc.
To check the last possibility, use a strong light and a magnifying glass to see if the hole appears blocked. If it does DO NOT try and clear it using a pin or probe as the microphone is directly behind it and you may damage it. Try a vacuum cleaner using short sharp bursts to see if this clears the obstruction.
If it doesn't appear to be blocked then see warranty claim above.