You can set your phone to be completely unreachable, for example you can set a certain time frame in which you wish to receive no calls. As soon as the DND function is activated all incoming callers will receive a busy tone.

How to:

• As administrator click Settings and change to the Settings page for the user whose phone you wish to enable DND on. Once you are logged in as said user click Settings.
• Click on Phone and then Advanced Settings
• On the following page change Do not disturb to Yes
  

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