Once the SIM card is removed from the iPhone, you can see numbers stamped on one side of the SIM card, opposite the metal contacts. These numbers contain various information about the wireless carrier and your account.
Up to 22 digits can make up this Integrated Circuit Card Identifier (ICCID). The ICCID is made up of several individual parts. For example, on an AT&T Mobility SIM card the first seven digits are the Issuer Identification Number (IIN):