This is considered unlikely, especially as some operators use their own version of A3. Keys Ki and the matching IMSI could be compromised by someone selling the information for money. Any mobile will take on the identity of a subscriber by insertion of a smart card. The mobiles now become attractive items to steal, as they can be used with another SIM card. Although to an operator, at first evaluation, it may seem as the stolen mobiles have no effect, as they do not affect a subscription, there will be problems with an increase in customer facing staff to handle esquires, and a possibility that GSM handsets are expensive to insure.
Grey lists are for local tracking of mobiles within a network.
GSM has defined a procedure so that approved, lost or stolen mobile IMEIs can be communicated to all other operators. All operators are able to post their black lists to the CEIR, and in return collect a consolidated list of all operators black and white lists. By this method stolen or invalid mobiles can be quickly barred throughout the world. Where it is possible to gain service without full credit checking, or verification of the identity of the customer. In some countries, credit checking is not available.