Do Not Disturb (DND) is sets your phone on silent mode and ignores incoming call notifications when you need to avoid distractions.
When you turn on DND, your incoming calls automatically goes to your voicemail. If you have auto-forward enabled, this will forwarded to the designated DID or extension.
For your GXP 16xx, you can enable or disable the DND using your device:
1. Press the menu button on your device
2. Select "Preference" by clicking the arrow down button
3. Select "Do Not Disturb"
4. You can choose either to Enable DND or Disable DND
5. The DND icon on the LCD display as shown below indicates that Do Not Disturn mode is enabled.
6. Pressing the Mute button when there is no active on-going call also enables or disables the Do Not Disturb mode.