i don’t know the quintum gateway. but if it does radius and / or syslog, they are free products you can find. most machine support several syslog servers, but you can have only one radius ‘active’. it is best to have at lest 2 radius servers (redundancy), but the devices do communicate with only one of them. so, if your billing system is allready using radius, you must find another way (like syslog). do a search, you will find many products. many in the linux world.
winradius is a good choice
(http://www.itconsult2000.com)
kiwisyslog for syslog(http://www.kiwisyslog.com/)
good luck