The Red Circle icon with a line through it () on the idle screen indicates that DND mode is activated.
You can press the Line soft key when the handset is idle to check the line status. The Red Circle Icon in the bottom of a line indicates that the DND mode is activated for this line.
The handset will reject all incoming calls to the line, which will make the LCD screen display the missed call icon (red, bouncing arrow) followed by the number of missed calls.