A method (100, 110, 120, 200) of establishing lawful interception, LI, of an LI target within a telecommunication network, the method comprising: at a communication device hosting an LI administrative function, ADMF: receiving on an LI Handover Interface 1 from a Law Enforcement Monitoring Facility, LEMF, a first request message containing: - a target identifier comprising a known identity of an LI target, wherein the known identity is for a first network operating using a first mobile network technology; and - a request for additional identities associated with the known identity; and sending on an LI X1 interface a second request message to a user register requesting additional identities associated with the known identity; at a communication device hosting the user register: receiving the second request message; obtaining from the user register at least one additional identity associated with the known identity; and sending a message to a lawful interception delivery function, LI DF, the message containing said at least one additional identity associated with the known identity obtained from the user register; and at a communication device hosting the LI DF: receiving the message from the user register on an LI X2 interface; and sending an intercept related information, IRI, message on an LI Handover Interface 2 to the LEMF containing said at least one additional identity associated with the known identity, wherein at least one additional identity of the additional identities is for another network operating using a different mobile network technology to the first mobile network technology. at a communication device hosting an LI administrative function, ADMF: receiving on an LI Handover Interface 1 from a Law Enforcement Monitoring Facility, LEMF, a first request message containing: - a target identifier comprising a known identity of an LI target, wherein the known identity is for a first network operating using a first mobile network technology; and - a request for additional identities associated with the known identity; and sending on an LI X1 interface a second request message to a user register requesting additional identities associated with the known identity; receiving on an LI Handover Interface 1 from a Law Enforcement Monitoring Facility, LEMF, a first request message containing: - a target identifier comprising a known identity of an LI target, wherein the known identity is for a first network operating using a first mobile network technology; and - a request for additional identities associated with the known identity; and - a target identifier comprising a known identity of an LI target, wherein the known identity is for a first network operating using a first mobile network technology; and - a request for additional identities associated with the known identity; and sending on an LI X1 interface a second request message to a user register requesting additional identities associated with the known identity; at a communication device hosting the user register: receiving the second request message; obtaining from the user register at least one additional identity associated with the known identity; and sending a message to a lawful interception delivery function, LI DF, the message containing said at least one additional identity associated with the known identity obtained from the user register; and receiving the second request message; obtaining from the user register at least one additional identity associated with the known identity; and sending a message to a lawful interception delivery function, LI DF, the message containing said at least one additional identity associated with the known identity obtained from the user register; and at a communication device hosting the LI DF: receiving the message from the user register on an LI X2 interface; and sending an intercept related information, IRI, message on an LI Handover Interface 2 to the LEMF containing said at least one additional identity associated with the known identity, wherein at least one additional identity of the additional identities is for another network operating using a different mobile network technology to the first mobile network technology. receiving the message from the user register on an LI X2 interface; and sending an intercept related information, IRI, message on an LI Handover Interface 2 to the LEMF containing said at least one additional identity associated with the known identity, wherein at least one additional identity of the additional identities is for another network operating using a different mobile network technology to the first mobile network technology.