Okay, lets say you may need some type of restriction level, Site A cor 1-10, Site B 11-20.
Your cor groups would like something like this
01 members 1,11-20
02 members 1,2, 11-20
03 members 1-3, 11-20
04 ...
11 members 1-10,11
12 members 1-10,11,12
13 members 1-,10, 11-13
14 ...
Then you create routes for each group like route 1 using cor 1, route 11 using cor 11. You then add them to a route list such that 1 or 11 are chosen. Your dialed digits point to the list and depending upon which site you are, you go out the one you can. The Mitel ARS is built upon routes, lists, plans. You don't see much plans any more but still comes into play if you need to restrict buy time of day or calls out a carrier is chaeper.