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This paper investigates the fraudulent usage of mobile telecommunications services due to cloned subscriber identity modules (SIMs). In GSM or the Universal Mobile Telecommunication System (UMTS), a mobile phone or mobile station uses its SIM to gain network access through authentication. If the SIM of a legal user is cloned, then the cloned SIM can be used for GSM/UMTS authentication and fraudulent usage may occur. Under the existing GSM/UMTS mobility management and call setup procedures, we show how quickly the fraudulent usage can be detected. We also propose a simple solution to speed up the detection of potential fraudulent usage.