 study from the Ferris Research Institute [1] estimates that in 2007 spam will cost US$100 billion worldwide, with US$35 billion in the U.S. alone. The same study also mentions that the percentage of spam messages sent daily is greater than 75%. Spam poses several challenges to both Internet users and regulatory agencies. It is typically anonymous, indiscriminate and global. With these characteristics spam has become a popular vehicle for promotions that may C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security be illegal, unscrupulous or use tactics that would not be commercially or legally viable outside the virtual environment. A report to the U.S. Federal Trade Commission (FTC) [2] estimates that roughly half of all unsolicited commercial e-mail contains fraudulent or deceptive content. Some of the key issues raised by spam include privacy, illegal/offensive content, misleading and deceptive trade practices and burdensome financial and resource costs. As the spam information is without interest for the majority of people to whom this information is sent, the spammers try to utilize widely used carrier tools that have extremely low cost in order to guarantee high gains. A few years ago, fax was the main tool used for sending spam, however the current favorite tools utilized for transmitting spam information are e-mails and phone calls. Considering Voice over IP (VoIP) spam (also known as spam over Internet telephony, SPIT), the costs for delivering spam messages are roughly three orders of magnitude cheaper than traditional circuit-based telemarketer calls. For the future, IP telephony seems to be the adequate means for sending spam because of the low cost of the Internet connection and the convergence of data and voice provided by the corresponding VoIP protocols. The Ferris Research Institute [1] predicted in April 2005 that VoIP spam would be a common problem within a year. Currently, there are very little telemarketing calls across international bor- ders, largely due to 