How do I add text message as two-step authentication?

In this article:

Text Message-

Text Message two-step authentication works by submitting a one-time security PIN sent to the phone number of your choosing. (SMS-enabled numbers only. Please note message and data rates may apply.) This pin is sent upon correctly submitting your account username and password credentials.

 

• Please enter an SMS-enabled phone number in the provided field and click the Add Text Message button.

  

• Then enter the pin in the provided field and click the Verify button.

        

 

Please Note: Text message two-step authentication is only available for United States and Canada telephone numbers.