If it's a fairly recent vintage system, you should be able to accomplish what you want by using "BLF ringing". As noted, all CO calls that ring at a phone will try to follow that phones forwarding. BLF buttons that ring (actually they beep) do NOT forward. There are lots of fun things to do with BLF ringing. I've setup quasi-virtual-phantom extentions by hanging a POS digital phone next to the switch and then assigning BLF buttons for it all over the place. They can ring,no ring,delay ring in both day and night modes.
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