Why screen get black while calling someone

iPhones are designed in this way. They have a proximity sensor on the screen and when pushed near the ear the screen will turn off. This saves the battery. Or does the screen go black when not near your ear? If it does this could mean the sensor is dirty, try giving the screen a good clean especially at the top near the sensor. If it is still doing it I would replace the front camera assembly as this has the proximity sensor on it. This is an easy process on the 6s plus.