A method of authenticating a request from a mobile station to conduct a communications session in a telecommunications system, the telecommunications system comprising: a plurality of radio access nodes (4, 14); a first authentication service for authenticating said mobile station for a first type of communications session, in which said mobile station is authenticated to communicate via a first radio access node of the plurality of radio access nodes, such that performance of the first authentication service serves to authenticate the mobile station in respect of communications sessions of the first type involving any radio access node of the plurality of radio access nodes other than the first radio access node; and a second authentication service for authenticating said mobile station for a second type of communications session, in which said mobile station is authenticated to communicate via a given radio access node, and wherein performance of the second authentication service is required for the second type of communications sessions involving radio access nodes other than said given radio access node, said first and second types of communications session each having an unauthenticated state and an authenticated state, said method comprising conducting data communication with the mobile station, during a performance of at least part of said second authentication service whereby said mobile station is to enter into an authenticated state for a new communications session of said second type, by conducting data communication via an existing communications session of said first type, when the mobile station is in an authenticated state for said existing communications session of said first type and wherein said data communication comprises receiving a message containing an authentication token from said mobile station, wherein prior to conducting said data communication with said mobile station and responsive to receipt of a registration request message from the mobile station (S2.2), said second authentication service rejects said registration request (S2.5) and thereafter monitors for receipt of predetermined data from the mobile station via the communications session of the first type (S4.1) so as to authenticate the mobile station to conduct a second type of communications session, said predetermined data comprising the authentication token, wherein said plurality of radio access nodes comprise cellular telecommunications network access nodes and said first authentication service is a cellular telecommunications network authentication service, and wherein said given radio access node comprises an unlicensed mobile access, UMA, access point and said second authentication service comprises an unlicensed mobile access, UMA, access point authentication service. a plurality of radio access nodes (4, 14); a first authentication service for authenticating said mobile station for a first type of communications session, in which said mobile station is authenticated to communicate via a first radio access node of the plurality of radio access nodes, such that performance of the first authentication service serves to authenticate the mobile station in respect of communications sessions of the first type involving any radio access node of the plurality of radio access nodes other than the first radio access node; and a second authentication service for authenticating said mobile station for a second type of communications session, in which said mobile station is authenticated to communicate via a given radio access node, and wherein performance of the second authentication service is required for the second type of communications sessions involving radio access nodes other than said given radio access node, said first and second types of communications session each having an unauthenticated state and an authenticated state, said method comprising conducting data communication with the mobile station, during a performance of at least part of said second authentication service whereby said mobile station is to enter into an authenticated state for a new communications session of said second type, by conducting data communication via an existing communications session of said first type, when the mobile station is in an authenticated state for said existing communications session of said first type and wherein said data communication comprises receiving a message containing an authentication token from said mobile station, wherein prior to conducting said data communication with said mobile station and responsive to receipt of a registration request message from the mobile station (S2.2), said second authentication service rejects said registration request (S2.5) and thereafter monitors for receipt of predetermined data from the mobile station via the communications session of the first type (S4.1) so as to authenticate the mobile station to conduct a second type of communications session, said predetermined data comprising the authentication token, wherein said plurality of radio access nodes comprise cellular telecommunications network access nodes and said first authentication service is a cellular telecommunications network authentication service, and wherein said given radio access node comprises an unlicensed mobile access, UMA, access point and said second authentication service comprises an unlicensed mobile access, UMA, access point authentication service.