llee, (b) the traffic patent filter evaluates the incom- ing calls received from a specific caller or ship domain SO as to ensure that they do not exceed the predetermined threshold, (c) the black and white list that is based on the black and white list filtering, (d) Bayesian learn- ing, every call is evaluated, on the basis of trusted information that is made available by third party entities, with regards to the behavior of the partic- ipating entities. The existence of trusted information is based on the pre- sumption that all participating entities have established prior calls, (e) social networks and reputation: this technique is used SO as to accept a call that is based on social relationships already established by the user in its VoIP environment. 19.5.1.11 VoIP SEAL VoIP SEAL [21] is a system that is divided into two main stages. The first stage uses various models which give a score between -1 and 1, the higher the score, the higher the probability of the call being SPIT. Moreover, two thresholds exist: On the one hand, if the score is inferior to the lower threshold, the call moves to a second stage that usually entails a CAPTCHA test. On the other hand, if the score is higher than the superior threshold the call is rejected. The main characteristic of VoIP SEAL is the adoption of a modular architecture that facilitates adding or updating of certain modules SO as to take defence measures against SPIT. 19.5.2 Anti-SPIT Mechanisms Classification Classification of anti-SPIT countermeasures in prevention, detection and han- dling is useful because it provides information about the scope. For example, it might be useful to combine prevention with handling mechanism, or detection with handling mechanism. In the following prevention refers to the mechanisms applied to avert SPIT, i.e., avoid a SPIT call reaching the callee domain or phone. This avoids com- munication and processing overheads, conserves costs and resources, and min- imizes the annoyance of the end-user. Detection means the identification of S