The Do Not Disturb feature on your phone will route all incoming calls to that extension to voicemail. To enable this feature on your phone, press the Mute/Microphone button as shown below (this button may be in a different location depending on your phone's model).
When Do Not Disturb is active, you will see the following Do Not Disturb icon on your phone.
To de-activate this feature, press the Mute/Microphone button again.