In order to access the Do not disturb setting, click on the "Do Not Disturb" tab (as shown in the picture below)
Do Not Disturb Switch - This will enable or disable the do not disturb option for the user, enabling this will stop anyone (excluding exception) from ringing this users phones
DND Exception List - This is where the user will set exceptions (users that are able to ring through to the user even when they are in DND)