Minor iPhone problems that don't resolve through restarting the device may benefit from removing the SIM card with a paper clip. Sometimes, the process of taking out and reinserting the SIM card will reset the phone and restore it to its previous functionality. You can remove the SIM card with a paper clip regardless of the iPhone model you are using. This method of resetting the iPhone can be particularly helpful for errors that read "no SIM card installed" or when iTunes can't complete the restore process.
Turn your iPhone off.
Locate the SIM card slot on your iPhone. Depending on your iPhone model, it may be along the side or on the top of the phone. In the iPhone 4, the slot is on the right side of the phone. It is on the top in all other models. It is easily distinguishable by the small pinhole within an oval "drawer."
Lengthen the paper clip so that you can use one end of the clip to open the sliding SIM card tray.
Stick one end of the paper clip into the hole and press firmly until the tray pops open.
Remove the SIM card from the tray.
Re-insert the SIM card in the same orientation as you found it. The card will only fit into the slot one way, so don't try to force it into the slot.
Turn your iPhone back on.