Before removing or inserting a SIM card, you must turn off your iPhone. Press and hold the Sleep/Wake button, then slide 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 or insert the SIM card tray using a paper clip or SIM eject tool.
Remove or insert the SIM card 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.
If a SIM is not in the device, "No SIM" will display at the top of the screen.