DND can be used to automatically reject calls on a Yealink handset and the callers will receive a busy message or if a divert has been set up so that when the handset is unreachable the call will follow the divert rules which are in place when DND is active.
TO ACTIVATE DO NOT DISTURB
While the phone is idle, press the DND soft key
The DND icon should then be displayed at the top right of the phone screen - 
Incoming calls will then be automatically rejected, and if the user receives a call while DND is active it will show as a missed call on the LCD screen as shown below:
TO DEATIVATE DO NOT DISTURB
Press the DND soft key again, the DND icon should no longer show at the top right of the LCD screen. Calls will now come through to the phone again.