Did you also have a look at this article? Troubleshooting iPhone hardware
- Remove the SIM card.
- Verify that the SIM card is not damaged or dirty, then reinstall the SIM card.
- Try turning iPhone off and then on again.
- Wait approximately two minutes, then see if iPhone detects the SIM card and if it will register on the network.
- If that doesn't work, try another SIM card for that carrier, if possible, to see if it is detected.
- If the message still exists, please go to: apple.com/support/iphone/service/faq/ for information on service.
Note: If you need to send your iPhone in for service or if you want to temporarily use a different phone, you may be able to use the SIM card inside your iPhone with another GSM phone.
This problem could also come up, if someone had modified software installed on his phone and tried to update the firmware after that.