Iwatsu ENTERPROSE-CS Owner's Manual page 203

 Press the Day-1, Day-2, Day-3, or Night key to change the ringing
mode. The new selection will display in reverse video and Mode
Change Successful will appear on the second line of the display
 Press Exit to return to the main display. The new ringing mode
selection will appear on the display
Menu Operations in System Ringing Mode (Forced Night)
When a system is in System Ringing Mode, pressing existing Group Night
Mode key from an Iwatsu ICON Series (5810, 5910, and 5930) attendant
position will take you to the Select Ringing Mode screen with limited
selections. The menu will prompt the attendant to select either In Forced Night
(Group Night) or In System Mode.
This operation has a Group Night override for system ringing. Incoming calls
through a specified trunk group will generate ringing based on the Night
ringing table
ATTENDANT POSITION USER GUIDE - 185