In this paper, the authors analyze a real-life application of virtualization: the Italtel Virtual Session Border Controller (VSBC). The measurements obtained in ad hoc loading experiments show that the VSBC performance is not linear with respect to variations in the call rate. Such a behavior is not in accordance with the theoretical results predicted by standard statistical tools based on queuing theory. As a consequence, particular attention must be paid to accurately assess the VSBC performance, because inaccurate estimates could lead to undue costs, or under-performing solutions. To overcome this problem, a novel approach to accurately predict the VSBC performance is proposed, which allows optimizing the system behavior and minimizing its costs.