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 activate DND feature on the SIP-T38G IP phone: 

1. Press the DND soft key when the phone is idle. 
The  icon on the idle screen indicates that the Do Not Disturb mode is enabled. 
Incoming calls will be rejected automatically and "n New Missed Call(s)" ("n" indicates the number of the missed calls) will prompt on the LCD screen.