A device (105, 502, 602, 801) for operating in a communication system that comprises a plurality of individual networks and each network (104, 113) in the plurality of individual networks being a mobile network, wherein: the device (105, 502, 602, 801) comprises at least two subscriber modules for assisting the device (105, 502, 602, 801) to attach to at least three networks (104, 113) of the plurality of individual networks, the number of those at least two subscriber modules being fewer than the number of the at least three networks (104, 113); and the device (105, 502, 602, 801) is configured to: before establishing a connection with one of the at least three networks (104, 113), initiate a pre-attach procedure by transmitting a multi-attach request to a subscriber server to attach the device to all of the at least three networks (104, 113), wherein the multi-attach request comprises an indication that it is a multi-PLMN attach message; wherein the device (105, 502, 602, 801) is configured to initiate the pre-attach procedure in dependence on triggering events including one or more of: switch-on of the device (105, 502, 602, 801); a determination that the device is approaching a border of a zone (110, 111, 112) that is assigned to a network of the plurality of individual networks different from a network (104, 113) that the device (105, 502, 602, 801) is currently attached to; and receipt of information from a network (104, 113) relating to network loading and/or a type of service that can be accessed through one or more networks (104, 113) of the plurality of individual networks; and connect to one of the at least three networks (104, 113) for communicating data with that network while remaining attached to the others of the at least three networks (104, 113). the device (105, 502, 602, 801) comprises at least two subscriber modules for assisting the device (105, 502, 602, 801) to attach to at least three networks (104, 113) of the plurality of individual networks, the number of those at least two subscriber modules being fewer than the number of the at least three networks (104, 113); and the device (105, 502, 602, 801) is configured to: before establishing a connection with one of the at least three networks (104, 113), initiate a pre-attach procedure by transmitting a multi-attach request to a subscriber server to attach the device to all of the at least three networks (104, 113), wherein the multi-attach request comprises an indication that it is a multi-PLMN attach message; wherein the device (105, 502, 602, 801) is configured to initiate the pre-attach procedure in dependence on triggering events including one or more of: switch-on of the device (105, 502, 602, 801); a determination that the device is approaching a border of a zone (110, 111, 112) that is assigned to a network of the plurality of individual networks different from a network (104, 113) that the device (105, 502, 602, 801) is currently attached to; and receipt of information from a network (104, 113) relating to network loading and/or a type of service that can be accessed through one or more networks (104, 113) of the plurality of individual networks; and connect to one of the at least three networks (104, 113) for communicating data with that network while remaining attached to the others of the at least three networks (104, 113). before establishing a connection with one of the at least three networks (104, 113), initiate a pre-attach procedure by transmitting a multi-attach request to a subscriber server to attach the device to all of the at least three networks (104, 113), wherein the multi-attach request comprises an indication that it is a multi-PLMN attach message; wherein the device (105, 502, 602, 801) is configured to initiate the pre-attach procedure in dependence on triggering events including one or more of: switch-on of the device (105, 502, 602, 801); a determination that the device is approaching a border of a zone (110, 111, 112) that is assigned to a network of the plurality of individual networks different from a network (104, 113) that the device (105, 502, 602, 801) is currently attached to; and receipt of information from a network (104, 113) relating to network loading and/or a type of service that can be accessed through one or more networks (104, 113) of the plurality of individual networks; and connect to one of the at least three networks (104, 113) for communicating data with that network while remaining attached to the others of the at least three networks (104, 113).