Here is a general overview of the free/busy architecture in Kolab:
According to /etc/cron.d/kolab-freebusy free/busy files are generated every fifteen minutes by the kolab-freebusyd daemon, which writes logs to /var/log/kolab-freebusyd.log as it does its work. The daemon is configured by the /etc/kolab-freebusy/config.ini file (at least on Debian).
The files are written by the daemon to /var/lib/kolab-freebusy/ and these are read by the kolab-freebusy Web service which is configured by the /etc/apache2/sites-available/kolab-freebusy site definition file (on Debian), presumably somewhere like /etc/httpd/conf.d/kolab-freebusy on CentOS and related systems. This file contains something like the following:
ScriptAlias /freebusy /usr/share/kolab-freebusy/public_html/index.php
Free/busy resources should be published to the indicated location - /freebusy if configured as above - and the Web service mostly wraps the generated files with some additional logic.