[VoiceOps] Billing of Forwarded Calls

So how do most of you deal with billing of forwarded calls (specifically where the calling number on the forwarded leg is using the original calling number from the inbound leg) in a SIP environment when the originally called number is not preserved in the new invite? In this case there is no way to match the calling or called number to a specific customer. Do you bill by IP address or interface instead? Do you somehow use a system that correlates the forwarded leg to the original inbound leg? I've come across this issue a few different times when trying to bill off of SIP messaging logs, for instance radius off a SIP SBC or SQL logs from SER. Thanks, -Scott