So I don't think you quite understand how "COR" relates to things... COR is just a value which in itself means nothing... What you have to find is the ARS Route the call is taking out the system, look at the COR Group assigned to it, then look at the COR Group Assignments to see what COR is in the group... and the old Mitel rule is "if you're, you're out" so if that COR value exists in the COR Group assigned to the Route, then the call is blocked.
"Internal Outbound calling is going straight to customer's voicemail."
This I don't quite understand... If the call is leaving the system, the fact it's going to voicemail is not relative to you unless the far end has a particular rule to block your number, is in night service or something of that like.