In-call audio. When you’re on a call, the Volume buttons control the volume of the other person.
Media. When you’re not on a call, the Volume buttons control media volume during playback or before you listen.
Phone ringtone, notification sounds, and alarms. To adjust these volumes, press a Volume button and then use this shortcut on your home screen:

, silent
, and vibration only
.To silence a ringing phone, press either the Power button or the Volume down button.
To switch to vibrate at any time, press & hold the Power + Volume up buttons until you feel a vibration and see
.
If you want to customize Power & Volume up so that it silences the phone instead of vibrating: