If you mean the sofia stack tracing ("sofia loglevel $component $level") then that by default logs as console level, you can adjust that with "sofia tracelevel debug" and then it should go to the logfile.
I'd advise against that though for anything other than debugging a problem - it is extremely verbose. That means extremely large log files and the amount of I/O may affect the performance of your system.
FreeSWITCH's debug-level logging is separate and should be checked first - the fs_cli for that is "/log debug"
-Steve