1. A network connection establishment method carried out by a user equipment in a mobile communication system, wherein the mobile communication system comprises plural access networks with different technologies and a plurality of mobility management entities, and wherein the user equipment has plural identities and is within a first of the plural access networks, the method comprising: receiving, via the first of the plural access networks, an indication message indicating whether a context of the user equipment is available at one of the plurality of mobility management entities; determining, based on the received indication message, whether the context of the user equipment is valid; selecting, based on the determining result, which one of the user equipment's plural identities is to be used for establishing a connection to the first or a second of the plural access networks; and transmitting the selected user equipment's identity to the one of the first or the second of the plural access networks, to which a connection is to be established by the user equipment.