A method and a proxy server for selecting an input server of an IMS communication network in order to register a terminal in the IMS communication network. Following receipt from the terminal of an SIP registration message, the proxy server obtains a value of at least one field of the SIP registration message, the field being representative of a characteristic belonging to the terminal, and selects an input server using the at least one value obtained. Then, the proxy server sends, to the terminal, an SIP redirection message including an IP address of the selected input server.