A Subscriber Identity Module (SIM) is
|“||the smart card inserted into GSM cellular phones. The SIM identifies the user account to the network, handles authentication, and provides data storage for basic user data and network information. It may also contain some applications that run on a compatible phone.||”|
When a subscriber removes the SIM card, it can be re-inserted into another phone that is configured to accept the SIM card and used as normal. Each SIM card is activated by use of a unique numerical identifier; once activated, the identifier is locked down and the card is permanently locked in to the activating network. For this reason, most retailers refuse to accept the return of activated SIM cards. Common providers that require SIM cards include AT&T Mobility, T-Mobile and Nextel.