A network broadcasts a list of multiple core networks. A mobile station MS selects one and sends an uplink message (e.g., ATTACH or ROUTING AREA UPDATE Request) with a new format detailed by these teachings. The body of the message has a data section and an identity section, and the identity section includes: a temporary identifier for a mobile station, a bit sequence indicating that a selected core network is identified in the uplink message and indicating type for the temporary identifier (e.g., Foreign or Random TLLI), and an indication of the selected core network. For the Foreign TLLI there is an extension field for the additional bits of the Foreign TLLI as compared to the Random TLLI, and the Foreign TLLI is derived from the mobile station's PTMSI.