The only true way to determine who your callers are, i.e., 100 callers calling 40 times each or 40 callers calling 100 times each, is to view your call records and view the originating ANI. I manage a large switched voice network and we are faced with similar problems. We search the historical Call Detail Records and learn how many times a particular caller placed a call on our network. Since these calls must also be billed, your billing system should also have the ability to accommodate your needs.