Before removing or inserting a SIM card, you must turn off your iPhone. Press and hold the Sleep/Wake button, then select and drag the Power icon to the right.
Note: A SIM card is required in order to use cellular services when connecting to the AT&T network.
Remove the SIM card tray by inserting a paper clip or SIM eject tool into the small hole on the right side of the device.
Remove or insert the SIM card from/into the SIM card tray.
Push the SIM card tray back in until it clicks into place and is flush with the side of the device.
Turn the phone back on by pressing the Sleep/Wake button until the Apple logo appears.