The SIM is a chip that contains contact information, wireless carrier service and security information. The GSM iPhone is the only model that uses a SIM card; the CDMA model doesn't use one. When your iPhone won't pick up the SIM card, a card error may appear on the screen. A "No SIM Card" installed error may also pop-up on the screen when you try to make an outgoing call or send an SMS message.
The SIM card is a small piece of hardware required for GSM iPhones to work. The SIM card functions as a memory chip and can be moved from one GSM phone to another. Occasionally, your iPhone might not pick up or recognize a SIM card. The cause of this problem varies; however, some troubleshooting guidelines may help identify the problem.
If the SIM card isn't activated with iTunes, your iPhone won't pick it up. Accidentally dropping the phone on a hard surface can reposition the SIM, which can result in errors. If you're in a low network coverage area, you may also experience SIM card problems. Other causes of SIM card errors include hardware failure such as freezing or crashing. Your iPhone may freeze as a result of hardware overload or software crashes. Hardware problems can diminish your iPhone's performance by limiting its ability to pick up the SIM card.
Connect your iPhone to your computer and make sure the SIM card is activated with iTunes. If it's already activated, use a small pointed object such as a paper clip to eject the SIM card from its tray and then replace it to realign it to the metal contacts in the tray. If the SIM card errors persist, press and hold the "Sleep/Wake" button. Drag the red slider to turn off the phone and then turn it back on to reset the hardware and possibly solve this problem. You can also try resetting the network settings from the "General Settings" tab. As a final troubleshooting step, restore your iPhone with iTunes. Keep in mind that restoring the phone clears saved data and settings, so back up your iPhone with iTunes before proceeding.
Tips, Recommendations and Warnings
If you are still experiencing SIM card problems, try a different SIM card. If you don't have another SIM card, contact Apple support. The SIM may need to be replaced. In some cases, the phone may require service and repair. If you schedule a repair, back up saved data and settings with iTunes.