By using a SIM card, a subscriber can easily change the phone itself without losing her phone book and most importantly, without having to change her phone number.
Early versions consisted of the whole fullsize (85 x 54 mm) smart card. Soon the race for a smaller telephones called for a smaller version of the card. The card was cropped down to 25 x 15 mm (as illustrated). Since the SIM card slot is standardized, a subscriber can change carriers and use his current phone with a new provider's SIM card. However, this is difficult in the United States; almost all U.S. GSM providers SIM-lock phones that they sell—i.e., electronically lock their phones so that they can only be used with the provider's own SIM cards. Some providers will unlock a customer's phone once she has fulfilled her service contract. Others, such as AT&T Wireless, will not unlock phones under any circumstances. AT&T Wireless not only locks its phones against its direct competitors, but even locks them against non-AT&T Wireless providers that have partnership agreements with the company.
The use and content of the card is protected by several levels of access codes. PIN is the every day access code for normal use of the phone. PIN2 is reqired to use special functions (like limiting outbound telephone calls to a list of numbers). PUK1 and PUK2 is used to reset PIN1 and PIN2 respectivly.
Japan's PDC system also specifies a SIM, but this has never been implemented commercially. The specification of the interface between the Mobile Equipment and the SIM is given in the RCR STD-27 annex 4.
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