As I think, the simplest config is Cisco gateway with TCL IVR installed on it in every region and billing system in center. As say richard, there are similar solutions from another vendors, but I know nothing about it.
IVR = Interactive Voice Response. It may be based not only on Dialog voice cards, but (for VoIP) just a s/w, for example Cisco IP-IVR. In solutions for calling cards IVR is responsible for DTMF input from caller, notifying subscribers on current balance and so on.