You can use the DND feature to reject the incoming calls automatically on the phone and the callers can hear a busy signal or a message depending on your server. 

To configure the DND key mode via web user interface: 

1. Click on Phone->Features->DND
The phone will beep softly every 30 seconds to remind you that you still have a call on hold. 
2. Mark the desired radio box in the DND Key Mode field. 
3. Click Confirm to accept the change. 

To activate DND in phone mode: 

1. Press Menu
2. Select Features->DND
3. Press Left or Right to select Phone Mode from the User Name field. 
4. Select Enable from the DND field. 
5. Press OK to accept the change. 
The icon on the idle screen indicates that the Do Not Disturb mode is enabled. 
Incoming calls will be rejected automatically and "n Missed Call" ("n" indicates the number of the missed calls) will prompt on the LCD screen.