One problem of internet telephone networks which is expected to create some problems such as email spams for telephone systems is SPITs (Spam Over Internet Telephone). The detection of SPITs in internet telephone networks must be done immediately, irrespective of content and prior to the initiation of a session. On the other hands, process of disruptive contacts is exactly same as the process of healthy contacts which this can make the detection of SPITs challenging. Therefore, by introducing a new method which concentrates on users� behavior and a look at susceptibilities existing in Voice over Internet Protocol (VOIP) which can lead to SPITs, present paper attempts to introduce an anti-SPIT system which is able to detect simultaneous disruptive contacts in signaling stage. Taking into account the susceptibilities leading to SPITs, the proposed system identifies users� suspicious behaviors, and following examining contact characteristics, using a response/challenge method allow only entrance of users who have made a normal contact. The evaluation of the proposed method is done using its simulations, and the resultant detection rate and incorrect alarm rate indicate the success of this method in preventing the entrance of SPITs into a network.

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