Are you having difficulty recognizing a micro-SIM card, Invalid SIM” or “No SIM Card installed” alert appears intermittently then you can fix the issue by upgrading to iOS build 5.0.1 named 9A406.Apple has recently written an article about this issue at the following link.
Apple suggests the solution by install this update via itunes after backing up user content and your other files.
Following is the process by apple to install this update.
Note: These steps are necessary only if you are experiencing these symptoms on an iPhone 4S.
1) Back up your user content and settings using iTunes or iCloud. See this article for more information.
2) Restore your iPhone 4S using iTunes.
3) After the process is complete, restore your user content and settings from your backup.
4) To confirm that you’ve successfully restored, verify that the version is now iOS 5.0.1 (9A406) in Settings > General > About.
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