Ringer Volume

I am assuming this is on a DTU 16D nec phone. On the right side of the phone is a volume up and down button. If someone calls you, you adjust this while the phone is ringing. If you adjust it after you answer it controls the voice volume (what you hear through the ear piece) if you adjust it while the handset is down but there is no one calling you it adjusts the display contrast on the phone.