I have already configured asterisk dispatcher with kamilio now I wants to configure freeradius with kamailio. What are the possible solutions available for VoIP Custom Billing ?
It depends on what you want to do (all AAA-features, or just accounting, etc). You may use radius modules, like ACC_RADIUS or MISC_RADIUS.
All the modules and their docs are here: http://kamailio.org/docs/modules/stable/