Are the ones you called or calling are uses different network or they are using Globe network? Weak signal can affect voice call even if both of you are using the same network. Make sure to check first your signal before receiving or calling. Another way to isolate the issue is try to go to an open space, then check if the voice clarity changes. You can also have your device check by authorized Samsung Service Center. Take note that if your device is no longer under warranty, any issue if your device that needs repair you will be charge for it.