Try using the earphones with the microphone attachment when making or receiving a call.
If the problem still exists there is a problem with the voice transmission circuitry in the phone.
Since the phone is only a few days old it will be covered by the manufacturer's warranty. Consult the warranty statement in the documentation that came with the phone as to what to do to make a claim for a warranty replacement.
If there is no problem when using the earphones and microphone attachment, there may be a problem with the phone's internal microphone. It may be either faulty or loose from the systemboard. Again since it is only a few days old it will be covered by the manufacturer's warranty. (See above regarding warranty).
There are some other possibilities:
1. The voice inlet hole at the bottom of the phone has a piece of clear tape covering it (factory installed) to protect it until it is removed by the user after purchase, muffling the voice input.
2. That the phone is in a case that may be covering the voice inlet hole, muffling the voice input.
3. The voice inlet hole at the bottom of the phone may be blocked with lint, (doubtful as it is new). Use a strong light and a magnifying glass to check to see if it appears blocked at all. If it appears to be, DO NOT use a probe (pin) to clear the hole as you might damage the microphone which is directly behind it. Try a vacuum cleaner, using short bursts and see if this clears the blockage.