The deployment of Next-Generation Networks (NGN) is a challenge that requires integrating heterogeneous services into a global system of All-IP telecommunications. These networks carry voice, data, and multimedia traffic over the Internet, providing users with the information they want in any format, amount, device, place or moment. Still, there are certain issues, such as the emerging security risks or the billing paradigms of the services offered, which demand deeper research in order to guarantee the stability and the revenue of such systems. Against this background, we analyse the security requirements of NGN and introduce a fraud management system based on misuse detection for Voice over IP services. Specifically, we address a fraud detection framework consisting of a rule engine built over a knowledge base. We detail the architecture of our model and describe a case study illustrating a possible fraud and how our system detects it, proving in this way, its feasibility in this task.