The AT&T GoPhone, which was previously Cingular, offers a pay-as-you-go plan, which means you do not have to have a contract for cellular service. To register your phone number and allow your GoPhone to be fully operational, you have to install a SIM card, which is a tiny card that contains a unique identifier to allow the AT&T service to locate you.
Turn the AT&T GoPhone off by pressing the "Power" button for several seconds.
Remove the back cover by applying downward pressure in the center of the cover. The cover should slide down and lift off.
Remove the battery by grasping its base on either side and lifting up and pulling back.
Align the notch of the SIM card with the bottom slot. The gold contact points should be facing down. Push the SIM card directly down into place.
Replace the battery just as it was removed, with the metallic contact points down and oriented toward the front of the phone.
Replace the cover by laying it over the back of the AT&T GoPhone and pushing it up toward the top of the phone.
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