How to Fix “No SIM Card Installed” Error on iPhone 4S by Installing iOS 5.0.1 Build 9A406

The recently released iPhone 4S specific build of iOS 5.0.1 resolves an ongoing SIM card issue that was effecting some users. The issue would cause some iPhone 4S’s to not recognize a SIM card, with the iPhone reporting a “No SIM Card Installed” or “Invalid SIM” error message, in addition to the phone not connecting to a cellular network.
To resolve the error, users have two choices:

• Backup and restore the iPhone through iTunes, then restore the iPhone from the last backup – easiest approach and recommended by Apple
• Download 9A406 directly and reinstall manually using the IPSW – more advanced

This makes the latest iOS 5.0.1 update a welcome change for iPhone 4S users who frequently encountered the problem, and this also answers the question as to what exactly the iPhone 4S specifc 9A406 build of iOS 5.0.1 aimed to accomplish. Thanks to MacRumors for discovering this
 
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