One disclosure of the present specification provides a method for a proxy-call session control function (P-CSCF) to obtain an identifier of a visited public land mobile network (V-PLMN) in order to register a user equipment (UE) roaming in the V-PLMN into an IP multimedia subsystem (IMS) network. The method may comprise the steps of: receiving a response message from an Interrogating-CSCF (I-CSCF) in response to the transmission of a REGISTER message for registering a user equipment into an IMS network; determining whether the received response message is a 401 unauthorized message; when the received response message is a 401 unauthorized message, transmitting the response message to the user equipment and generating an interface for transmitting and receiving data to and from an entity of an EPC network; and receiving an identifier of the V-PLMN from the entity of the EPC network.