A mobile communications system comprising: a base station adapted to form one or more cells, and to communicate with a mobile station; a gateway adapted to communicate with the base station; and a core network device adapted to communicate with the gateway, wherein the base station is configured to transmit a first message including: identification information of the base station; and one or more parameters indicating an environment of the base station, wherein the gateway comprises: receiving means for receiving the first message; storing means for storing correspondence information including: a first cell identifier in association with at least one of the identification information and the one or more parameters; determining means for determining the first cell identifier, corresponding to a cell of the base station, based on the correspondence information and the first message, and upon receiving a second message including a second cell identifier from the base station, generating the second message, including the determined first cell identifier in place of the second cell identifier; and transmitting means for transmitting the second message to the core network, and wherein the core network device is configured to receive the second message from the gateway, and, if the second message comprises information indicating an emergency call, to perform a search for a destination of the emergency call based on the first cell identifier and to store a result of the search, and if the need for the search for a destination of the emergency call based on the first cell identifier arises, the core network device is configured to skip the search using the result of the search stored in the core network device. a base station adapted to form one or more cells, and to communicate with a mobile station; a gateway adapted to communicate with the base station; and a core network device adapted to communicate with the gateway, wherein the base station is configured to transmit a first message including: identification information of the base station; and one or more parameters indicating an environment of the base station, wherein the gateway comprises: receiving means for receiving the first message; storing means for storing correspondence information including: a first cell identifier in association with at least one of the identification information and the one or more parameters; determining means for determining the first cell identifier, corresponding to a cell of the base station, based on the correspondence information and the first message, and upon receiving a second message including a second cell identifier from the base station, generating the second message, including the determined first cell identifier in place of the second cell identifier; and transmitting means for transmitting the second message to the core network, and wherein the base station is configured to transmit a first message including: identification information of the base station; and one or more parameters indicating an environment of the base station, identification information of the base station; and one or more parameters indicating an environment of the base station, wherein the gateway comprises: receiving means for receiving the first message; storing means for storing correspondence information including: a first cell identifier in association with at least one of the identification information and the one or more parameters; determining means for determining the first cell identifier, corresponding to a cell of the base station, based on the correspondence information and the first message, and upon receiving a second message including a second cell identifier from the base station, generating the second message, including the determined first cell identifier in place of the second cell identifier; and transmitting means for transmitting the second message to the core network, and receiving means for receiving the first message; storing means for storing correspondence information including: a first cell identifier in association with at least one of the identification information and the one or more parameters; determining means for determining the first cell identifier, corresponding to a cell of the base station, based on the correspondence information and the first message, and upon receiving a second message including a second cell identifier from the base station, generating the second message, including the determined first cell identifier in place of the second cell identifier; and transmitting means for transmitting the second message to the core network, and