METHODS AND APPARATUS FOR REGISTERING AN IMS SUBSCRIBER USING TEMPORARY IDENTIFIERS

A method of registering a subscriber in an IP Multimedia Subsystem (IMS), said subscriber being associated with an IMS Private User Identity (IMPI) and an IMS Public User Identity (IMPU) and further being associated with a 4G or 5G conforming User Equipment (UE). The method comprises: at the UE, generating a Temporary IMS Private User Identity (TIMPI) and a Temporary IMS Public User Identity (TIMPU) from a Globally Unique Temporary Identifier (GUTI) allocated by and received from a 4G or G network to temporarily identify the UE; sending an IMS registration request containing the TIMPI and the TIMPU from the UE to the IMS; at the IMS, using the TIMPI and the TIMPU to determine the GUTI, and communicating with the 4G or 5G network to determine, using the GUTI, an International Mobile Subscriber Identifier (IMSI) associated with the UE; at the IMS, using the IMSI to determine said IMPI and IMPU; and completing IMS registration of the subscriber using the IMPI and the IMPU.