If the phone is set to DND - Do Not Disturb, all incoming calls will be stopped and all calls will go to the voicemail or simply hang up (depending on how the extension has been set)

 

The below example is a using a Fanvil Phone (your screen may be different)

DND is SET (phone cannot receive incoming calls)

DND is NOT SET (phone will function as normal)

If you wish to action DND on or off on the phone.

Simply press the button underneath the option DND