eedom of communications. Filtering and screening of com- munications content for the purpose of detecting SPIT, without the consent of the communicating parties, should be considered as intruding fundamen- tal rights and therefore unlawful. In any case defining SPIT by content is related with the risk of infringing freedom of expression and introducing a kind of censorship. Especially some techniques of communication screening, © 2009 by Taylor & Francis Group, LLC SIP Vulnerabilities for SPIT, SPIT Identification Criteria like blacklisting, can raise questions in relation to the freedom of expression and freedom of information [5]. Filtering can also result in the blocking of legitimate information (the so-called false positive), which has as consequence the infringement of the freedom of expression [2,5]. Blocking spam by technical means could intrude on the right to informa- tional self-determination of the communicating parties. By definition some anti-SPIT modules need information from the user, such as his preferences, to be able to work [22]. Mechanisms based on the analysis of behavior and reactions (such as the challenge-response mechanism), the caller's character- istics reputation or the use of black/white lists imply the collection and use of personal data, i.e., any information relating to an identified or identifiable individual. Phone numbers allow indirect identification of subscribers through the use of reverse directories as well as through electronic communications ser- vices providers. Indirect identification is also possible through IP addresses, which can be traced back to a computer and through the provider conse- quently to a subscriber. Even if the link between subscribers and users is, in the case of IP address, less strong than by e-mail addresses and phone num- bers, most IP addresses can be tied to a log-in and may qualify as personal data [2]. This challenge-response mechanism can have as result the processing of personal data of SPITters to the extent that these data can be consider