We have a PRI, under ILG I have 2 analog and 2 ISDN group assignments. Im not a phone guy just pretending. Does that answer your question on type of CO lines?
Im not familiar with centrex feature. I remember seeing @F9 associated with a mailbox before our ES card took a dump so that's probably the one to use. Its not working though, when I dial the ext, it just goes to the main system greeting.
Some more info on the current setup: I have a pilot dn for the ext4378 which is set to mailbox 4378 with @F9-(phone#)H in the ext field.
