s users. Furthermore, the lack of integrity mechanisms allows mali- cious users to manipulate SIP signaling data (e.g., man-in-the-middle, MITM, attack) and tamper them (e.g., with SQL code). On top of the privacy and integrity issues VoIP providers should also take into account the client or/and service masquerading threat which was not applicable to PSTN. In the first case, the malicious user tries to act on behalf of a legitimate user (victim) by sending SIP requests that identify the victim's ID instead of the malicious user's one. For example, the malicious user may replace his ID with the victim's ID in the SIP REGISTER message that is used during the registration procedure of every SIP VoIP based service. During a service masquerading incident, a malicious user responds on behalf of the service, taking advantage of the fact that the SIP server is not properly authenticated by the other side and thus the attacker gains access to call information without being identified. Another important threat for all telephony services is toll fraud. Without doubt, all users expect accurate billing procedures. In the PSTN a malicious user could make free calls, without affecting a legitimate user's billing account. This was achieved by notifying, through specific tones, the call center that the call had been finished while the conversation was actually still in progress [11]. Similar incidents can occur in SIP based VoIP services, affecting, in financial terms, not only the provider but also the legitimate users [12]. With the continuously increasing demand for VoIP services, service corrup- tion is considered by most VoIP service providers as the most severe threat they have to cope with. Similarly to other Internet applications and services, in a SIP-based VoIP service a malicious user may exploit implementation errors or generate a large number of SIP requests (e.g., SIP INVITE, SIP REGISTER) in order to cause DoS and/or gain access in the VoIP provider's core network [3,6,13]. Furthermore, a malicious user can easily