Well a couple of things come to mind. Set the CO preset forward only to 450 in nite mode. Program a nite service button, usually on sta 100. This way the phones will ring wherever they are programmed during the day, and directly to VM when in nite mode. Another way is do the same thing with the night serivce as above, but program the CO preset forward time to 60 or 90 seconds before going to 450. If you use the night service button on a station you should disable DND for that station.
Steve