There are plenty of RADIUS servers available (and most of them are free). For instance FreeRADIUS http://www.freeradius.org/, OpenRadius http://www.openradius.net/, xtradius http://xtradius.sourceforge.net/, Radiator http://www.open.com.au/radiator/, …).
But note that just having a RADIUS will allow you to accept accounting requests from gateways and store them into the file/database/… You still need some engine to process this data and rate the calls. And this is the part you can either try to develop yourself or buy a billing software (usually comes with the the RADIUS server included).