Can you please enlighten on how billing server is to integrate with IVR? I mean when a call is terminated to IVR if it has to check billing/credit eligibility before transferring the call to an agent/terminating user.
If your billing server supports Diameter for real time billing, you can use Restcomm jDiameter in your application to connect to your Billing System and have the IVR being notified if the user runs out of money and cut the call.
If your billing server supports HTTP then you can do the same thing via REST API calls to your Billing server without any additional software