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SPam over Internet Telephony (SPIT) is expected to become a very serious issue in the next years. The threat is going to spin out from the well known email spam problem by bot nets being re-programmed to initiate not just spam emails but also Voice over IP (VoIP) calls. Preventing SPIT is a new problem, because many well-established methods for blocking email spam cannot be applied. Currently, several SPIT prevention methods are being proposed but SPIT prevention research is still at a very early stage. In this paper, we propose an innovative way to detect SPIT calls by comparing applying hidden Turing tests that compare them with typical human communication patterns. For passing these tests, significant resource consumptions at the SPIT generating bot nets would be required which contradicts the spammer's objective of placing as many SPIT calls as possible, The proposed method has several advantages compared to other methods that also interact with the caller. We validated its feasibility with a prototype implementation that we integrated into our modular VoIP security system called VoIP SEAL.