Spam over IP Telephony (SPIT) is expected to become a serious problem as the use of VoIP grows. This kind of spam is appreciated by spammers due to its effectiveness and low cost. Many anti-SPIT solutions are applied to resolve this problem but it is still limited in some cases. We propose in this paper a SPIT detection algorithm based on user's call behavior. Simulation results show the efficiency of our detection method and outline the most significant parameters. We find that the inspection of call duration allows to quickly and precisely discover SPIT messages.