COR Can be difficult to understand.
The way i learned it is to remember the saying "If you are in, You're out"
Routes have "COR Groups"
User's have "COR's"
Check the user's "COR" lets say it's "5"
The trick is to follow the ARS Digits dialed, To find out what "Route" the call would take, The route will have a "COR Group" applied to it. I'll use "COR Group 10" for my example.
Once you have the COR and the COR Group, this is where the "If you are in, You're out" part comes in.
Check under System Properties -> System Feature Settings -> Class of Restriction Groups.
Look at group 10, If group 10 has COR 5 in it, The call is blocked. If 5 is not in the group the call is allowed.
Keep an eye out for ranges
So if group 10 says:
1-10 (COR 5 is in the group) Call Blocked
1,2,4,9-32 (COR 5 is not in the group) Call Allowed.
Hope this explanation helps !
I struggled with COR when i first learned Mitel.