I have had good experience using Voicemaster gear for prepaid calling cards, we are runing prepaid calling cards in Northern Virginia and Canada.
http://www.procomm100.net and you can also see http://www.raniphone.com the Voicemaster is a h323-sip gatekeeper that can be configured Proxy, Static or routed mode and has a complete Radius/Gatekeeper based solution integrated to a Sybase database on a Http graphic user interface.
I think is the best you can get in the market for the price.
You can contact me if you need more information.