Ever since iPhone 4S is released, many bugs have been found in this device. First of all the Poor Battery Life issue irritated many users, than many were experiencing audio echoing during Phone Calls. Some users were having issues while connecting with Wi-Fi or 3G.
Well, now another issue has just hit the Apple's discussion board. According to the one support forum many users are having “No Sim Card installed” issue often even SIM card is inserted in their iPhone. This issue can be temporarily fixed by restarting the iPhone. If you still have the same issue, you have to replace your SIM from your carrier and swap out the old one.
This issue hasn't been confirmed whether it is a software bug or a problem in the hardware. And it is also not confirmed that it may be due to a faulty sim cards shipped with their iPhone 4S.