Given the pager's limited message space, American teens have devised a special numerical code.

BBC: The pager rings off

When users scratch off the labels, they see a numerical code.

FORBES: Tech Startup Uses Cell Phones To Root Out Counterfeit Drugs

Gmail service offers an option for users to access their email accounts using both a password, and a numerical code separately sent to the authorized user's mobile phone.

WSJ: False Associated Press Twitter Message Sparks Stock-Market Selloff