asterisk users - May 2006 - Billing when forwarding incomming calls from SIP phone

Hi All

Does anyone know how to track calls that come in from the PSTN, go to 
the user SIP phone and then get forwarded from the user SIP phone back 
out to PSTN (typically the users mobile number)?
It is a headache trying to bill the users who do this.  There is only 
the src PSTN number and dst forwarded number in the CDR.  No SIP details 
at all.  How do we track this?
Billing for standard calls from the SIP user to the PSTN is fine.

Regards
Garth