It's been a few months since I've worked with the osmocom CDR's, but from memory I think what you're looking for is the cdr configuration option. In osmocom, "log" refers to the logs of the actual programs (think what you would see in syslog). I've only ever used cdr's in the SGSN to track data usage for billing, but I believe you can get the same information on the circuit switched side of the world too. In the SGSN, the relevant config line is detailed in the latest SGSN manual
http://ftp.osmocom.org/docs/latest/osmosgsn-usermanual.pdf , although suspiciously the cdr config options are not listed under the SGSN vty reference anymore.
-Matt J.