A method and UE for registering with a third network node using IMS, the method creating a tunnel; authenticating a first public identity associated with the UE to the first network node; receiving configuration information with a second private identifier and a second public user identifier, and registering with a third network node using the second private identifier and the second public user identifier. Further, a method and first network node configured for authentication between a UE and a third network node using IMS, the method establishing a tunnel; authenticating a first public identity of the UE; receiving a configuration information message from the UE including a network identifier for a network the UE is registered on; obtaining, from a second network node, a second private identifier and second public user identifier; and providing the second private identifier and second public user identifier to the UE.