What do the numbers on a SIM card mean?

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):

89 - the Major Industry Identifier (MII) - 89 means telecommunications
01 - country code for the US
410 - the SIM card issuer identifier number (AT&T)

The remaining numbers consist of personalized account information and a check digit. Major carriers have the following issuer identifier numbers:

AT&T (030, 150, 170, 410, 560, 680)
Sprint (120)
T-Mobile (026, 160, 260, 490)
Verizon (004, 005, 012, 480)

The ICCID can also be viewed directly from iOS. Navigate to Settings -> General -> About and scroll down to ICCID.