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A subscriber identity module (SIM) is a smart card securely storing the key identifying a mobile subscriber. SIMs are most widely used in GSM systems, but a compatible module is also used for UMTS UEs and IDEN phones. The card also contains storage space for text messages and a phone book.

SIM card also can provide additional services for the mobile user. These include Global Roaming, live updates for stocks/shares, weather, useful information,etc.

By using a SIM card, a subscriber can easily change the phone itself without losing his or her phone book and, more importantly, without having to change her phone number.

The use and content of the card can be protected by use of PIN codes. One code, PIN1, can be defined to control normal use of the phone. Another code, PIN2, can be set, to allow the use of special functions (like limiting outbound telephone calls to a list of numbers). PUK1 and PUK2 is used to reset PIN1 and PIN2 respectively.

The SIM is also a database — it stores network state information such as its current location area identity (LAI). If the handset is turned off and back on again it will take data off the SIM and search for the LAI it was in. This saves time by avoiding having to search the whole list of frequencies that the telephone n

International Mobile Subscriber Identity code or IMSI

Each GSM mobile subscriber's SIM is assigned a unique 15 digit IMSI (International Mobile Subscriber Identity) code. This IMSI allows any mobile network to know the home country and network of the subscriber.

To check your IMSI number try the IMSI Number Analysis @ http://www.numberingplans.com/index.php?goto=imsi