E-mail spam protection is a topic of already intensive research, with solu- tions available based on simple white/blacklist or advanced Bayesian filters that have a high rate in detecting this kind of spam. Until now, close to no work was done in the area of detection and prevention of VoIP spam in gen- eral and specifically, SIP. While some of the experience gained from the work dedicated to current web and communication systems will be surely reused for the protection of VoIP systems, further work needs to be dedicated to exam- ine the specific possibilities of spam calls on VoIP systems. The main differ- ence between e-mail and VoIP spam lies in the fact, that voice is an "instant and real-time" communication tool, while e-mail is a relayed one. This allows e-mail spam detectors to analyze messages in before they are received by the client, while this is not possible with voice communication. Hence, new and innovative solutions are needed to counter the VoIP spam threat. 23.2 Related Work In order to fight the SPIT phenomenon, several anti-SPIT mechanisms have been proposed SO far. Some of them appear to be the basic blocks-modules of more sophisticated anti-SPIT architectures that have been proposed in the lit- erature [6-9]. Nowadays, the most important techniques are: black, white and gray lists [10,11], content filtering, challenge/response schemes, consent-based approaches, reputation-based mechanisms [12,13], SIP addresses management and charging-based mechanisms [14]. In fact, the above mentioned anti-SPIT mechanisms were evaluated based on some quantitative and qualitative criteria [15]. Accordingly, it is argued that the effectiveness and applicability of the proposed anti-SPIT mechanisms are judged as not adequate especially that they fail to address the majority of the evaluation criteria. More specifically, most of them try to satisfy only some straightforward criteria of anti-SPIT identification and handling, while they ignore more sophisticated ways of handle SPIT such as the usability, and the o