Welcome to Android Central! Can they hear you on speakerphone, or using a wired or Bluetooth headset? If so, then your primary phone mic is either blocked (e.g., by a case or by debris) or faulty. Inspect the mic opening carefully -- if it looks clogged with debris, use a needle and carefully try to remove some of it. If the mic opening looks clear, then the mic itself may be faulty -- in this case, bring it to a repair shop.