When you activate a phone that requires a new SIM card with AT&T, you will need to provide a SIM card number and the phone's IMEI number. The SIM card ties the phone to your billing account, while the IMEI number is the specific identification number for your GSM handset. The IMEI number is printed on the back of the phone under the battery cover, but the fastest way to find it is through the phone menus.
Contact AT&T to update your SIM card. You can go into an AT&T store, or call customer care by dialing 611 from your AT&T phone. If you bought your phone used, this is your only option.
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Look for the email containing the IMEI/SIM update link if you bought your phone new. AT&T sends an email to the account holder's email address they have on file when they purchase an upgraded phone.
Open the link in the email, and follow the on-screen directions to update the SIM card and IMEI number on your AT&T account.
Locate the IMEI number by dialing *#06# on your phone. The SIM card number is printed on the SIM card.