Microphone

Welcome to Android Central! Can they hear you on speakerphone, or using a wired or Bluetooth headset? If so, then your primary phone mic (at the bottom of the phone) is either faulty or blocked. Inspect the mic opening to make sure it isn't clogged by debris.
 
there could be different reasons. your phone case might be covering the mic. Try making test calls without the case. you could also check if the software you have is updated. if it isn't, then you should update immediately. in case the problem persists, it would be best to take your phone to the seller. have them check out what the problem is.