A method of authenticating a mobile wireless terminal 9 in a mobile telecommunications network, the terminal 9 being a subscriber of an Internet Service Provider (ISP) 11 rather than of a mobile network. The terminal 9 is allocated an International Mobile Identity (IMI) which is transmitted from the terminal 9 to a mobile network when the terminal 9 wishes to register with the network. A country code part and an operator ID part of the IMI are used by the network to determine the Internet Protocol (IP) address of the terminal's home ISP 11, which is then contacted by the network over the Internet to authenticate the mobile terminal 9.
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