Looking for a billing system that imports CDR records from providers and import to a database. Then run reports with DID and IP Source filters to support many customers using the same data.
inbound filters with DID so each customer has a DID table.
Outbound filter using the source IP address to tag the customer.
Nothing really out there exists for this, as far as I know. You would have to write your own to deal with your providers CDRs and then compare them to Asterisk CDRs.