A mobile communication system that utilizes multiple access technologies achieves multiple session registrations by deriving a plurality of extended unique device identifications from a specific unique device identification (e.g., private user identification (PIID) stored on a subscriber identity module (SIM)) assigned to a user equipment. Each of the plurality of extended unique device identifications have the benefit of allowing multiple registrations with one or more access networks while allowing a home subscriber system to detect the one unique device identification embedded in the extended unique device identifications for authentication purposes. Thereby, a large population of deployed UEs and access network infrastructure may benefit without replacement by allowing a UE to maintain session continuity when transitioning between access networks, to select a preferred access technology when in overlapping coverage areas without session interruption, or to maintain multiple sessions (e.g., simultaneous Voice over IP (VoIP) and media streaming) with different access networks.