Usually the server which collects the CDR is connected to the router if the billing center is there in different LAN and then the CDRs will be transfered to the billing center via FTP or FTAM protocol.
This how normally we tranfer the CDRs.
Depends on the vendor that is used. Better to have a Mediation device that can take CDRs from each NE and “stream” this towards the IT environment for onward processing (Fraud, Analytics, Campaigns, etc..).
Now (S)FTP is a very popular way to obtain CDRs from NE nodes.
FTAM was OSI’s version of an application for File Transfer.