VoIP (Voice over Internet Protocol), is becoming increasingly popular nowadays. Spam over Internet Telephony (SPIT), as one of the main vulnerabilities of VoIP systems, is becoming an interesting source of annoyance for spammers. Transmitting bulk unsolicited messages and calls in a VoIP system could be facilitated with the use of an automated procedure (i.e., bot). As the effectiveness of legal actions in preventing these attacks is limited, various ANTI-SPIT technologies have been developed. Some approaches try to detect a sender as a spammer while he is attempting to generate calls and messages, and some others try to establish an infrastructure which makes the system unlikely for a spammer, so they prevent SPIT. Selecting one of these two approaches, (Detection or Prevention) could certainly move large volume of market share between companies. In this paper, after surveying different ANTI-SPIT solutions, a competition between two VoIP providers is proposed. Game Theory method is used to simulate this competition. Results illustrate that apart from the scientific efforts made to prevent SPIT, the VoIP providers trying to maximize their profit, prefer to use detective approaches instead. Keywords; SIP; SPIT; Game theory;

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