A system for ensuring that a first subscriber in a unlicensed mobile access system is prevented from unauthorized use of a second subscriber's identity when the first subscriber requests services from a mobile station to a core network. The system comprises authenticating means for authenticating a mobile station to an unlicensed mobile access network before higher-layer messages are sent from the mobile station, and thereafter, for authenticating the mobile station to a core network. The system also includes a mobile station with a subscriber identity module that includes an identifying means for identifying the mobile station. The system further includes a unlicensed mobile access network for connecting the mobile station to the core network and for relaying signals between the mobile station and the core network. The unlicensed mobile access network comprises checking means for ensuring that higher-layer messages passing through the unlicensed mobile access network to the core network are from the mobile station that authenticated with the unlicensed mobile access network. The system also includes the core network for implementing transaction control and user services. During authentication between the mobile station and the unlicensed mobile access network, the unlicensed mobile access network stores the identifying means that is associated with the mobile station and after authentication between the mobile station and the core network, the unlicensed mobile access network uses the checking means to examine higher-layer messages that are sent from the mobile station to the core network and to ensure that the identifying means associated with mobile station is stored in the higher-layer message.