Can they hear you on speakerphone? If not, bring the phone back to where you bought it.
If they can, look for the little hole in the bottom - that's the mic hole. See if there's any lint or dust in it. (You pick up all sorts of stuff in there when you keep the phone in your pocket.) If there is, use a small sharp wooden toothpick to clean it out. (Dust and lint are good sound insulators). If not, bring the phone to your carrier for help. Something may be wrong at their end or the phone may be defective.