Cisco Unified Communications Manager is not designed to process the CDR records generated by the router during fallback. Calls that are placed through the PSTN receive billing information only by the PSTN billing system. You can use CiscoWorks Voice Manager to export the call history log to a file that can then be processed in CDR format. CDR records can also be logged onto a syslog or a RADIUS server.

The SRST router provides CDRs in two formats -- syslog and RADIUS. In both formats, you need an external server to collect the CDR records from the SRST router.

http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/vcallcon/ps2169/prod_qas0900aecd8028d113.html