Sorry I'm not that familiar with this phone system yet. (Most of my background is Vodavi/Vertical) I'm not quite sure what your looking for as far as system forwarding rules.

Under:
System > Devices and Service Codes > Stations > 177 > Forwarding Paths : Forwarding paths = 1, Description = FWD X 100

The call flow is as follows
Call comes in to PBX is answered by the Auto Attendant, you dial the Extension in question 177, and it goes directly to voicemail.