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 or before playback.
Phone ringtone, notification sounds and alarms. To adjust these volumes, press a Volume button and then use this shortcut on your Home screen:




To silence a ringing phone, press either the Power button or the Volume down button.
To switch to vibrate at any time, press and hold the power + Volume up buttons until you feel a vibration and see 
If you want to customise Power and Volume up so that this silences the phone instead of switching to vibrate:
You can place your phone face down on a flat surface to silence incoming calls and alarms, and to turn vibration on.
To enable this gesture:


