A telephone that is capable of accessing a memory for storing subscription information that includes a phone number, the telephone comprising: a selecting unit configured to select one of two Universal subscriber identity modules, USIMs (111, 112), the selected USIM being stored in the memory as first subscription information, the first subscription information being associated with a first network operator, being used to attach to the first network, and including a first phone number; a searching unit configured to search the memory for second subscription information that is different from the first subscription information, the second subscription information being associated with a second network operator and including a second phone number; a bootstrapping unit configured to perform, over a communication network of the first network operator, a Generic Bootstrapping Architecture, GBA, bootstrapping procedure toward a Bootstrapping Server Function, BSF, of the second network operator using the second subscription information; an obtaining unit configured to obtain the first phone number included in the first subscription information, and configured to obtain from the second subscription information an identifier, NAF_ID, and a key, Ks_NAF, of a call transfer server of the second network operator, and a Bootstrapping Transaction Identifier, B-TID, that has been generated as a result of the GBA bootstrapping procedure, the call transfer server acting as a GBA Network Application Function, NAF, and the B-TID enabling the call transfer server to identify the second phone number with support from the BSF; and a sending unit configured to send a call transfer request, the B-TID, and authentication information generated based on the Ks_NAF to the call transfer server, the call transfer request being for requesting the second network operator to transfer a call addressed to a phone number, which is identified by the call transfer server based on the B-TID, to the first phone number, wherein the call transfer request is authenticated by the call transfer server based on the authentication information. a selecting unit configured to select one of two Universal subscriber identity modules, USIMs (111, 112), the selected USIM being stored in the memory as first subscription information, the first subscription information being associated with a first network operator, being used to attach to the first network, and including a first phone number; a searching unit configured to search the memory for second subscription information that is different from the first subscription information, the second subscription information being associated with a second network operator and including a second phone number; a bootstrapping unit configured to perform, over a communication network of the first network operator, a Generic Bootstrapping Architecture, GBA, bootstrapping procedure toward a Bootstrapping Server Function, BSF, of the second network operator using the second subscription information; an obtaining unit configured to obtain the first phone number included in the first subscription information, and configured to obtain from the second subscription information an identifier, NAF_ID, and a key, Ks_NAF, of a call transfer server of the second network operator, and a Bootstrapping Transaction Identifier, B-TID, that has been generated as a result of the GBA bootstrapping procedure, the call transfer server acting as a GBA Network Application Function, NAF, and the B-TID enabling the call transfer server to identify the second phone number with support from the BSF; and a sending unit configured to send a call transfer request, the B-TID, and authentication information generated based on the Ks_NAF to the call transfer server, the call transfer request being for requesting the second network operator to transfer a call addressed to a phone number, which is identified by the call transfer server based on the B-TID, to the first phone number, wherein the call transfer request is authenticated by the call transfer server based on the authentication information.