You can use DND to reject incoming calls automatically on the phone. Callers will receive a busy message and can leave a voicemail if applicable.

To enable DND, press the red DND button at the bottom of the screen. Now all incoming calls will be rejected automatically and the screen will display "# New Missed Call(s)"  will appear on the touch screen.  You can see in the image below, the DND icon displays at the top of the screen, indicating it is ON. 

Just press DND again to remove it!