what you want is a softswitch that acts like a signalling proxy. by this I mean that it only passes messages and does not need to host the required bandwidth for each call. This way, there is no need to have bandwidth capacity to host the incoming and outgoing sides of the call if you just point the two end devices to eachother directly. check out http://www.qtelnet.com because the softswitch they just released is based on this feature and they let you get a free two port version. Contact peter, he is the sales rep for international.
the switch they have also has a prepaid calling card plugin and generates CDR’s and supports SIP->H323, h323->h323, SIP->SIP and h323->SIP with no additional configuration.
good luck with your business.