If we upgrade a carrier unlocked iPhone 3G or 3G S to iOS 4.2.1 firmware or any other version it will lose its activation and become unusable. Those people who bought unlocked iPhone models will get into trouble if they upgrade to new iOS without checking for the availability of compatible jailbreak/unlock tools. The iPhone will then keep itself locked at the “No SIM card inserted” message and with a connect to iTunes screen. And another message ” The SIM card inserted in this iPhone doesn’t appear to be supported” when we connect the device to iTunes. That means you have to get the original carrier SIM card to activate the iPhone 3G or 3G S again. But you can override this using activation of hactivation tools designed for the specific iOS version. Thanks to the iPhone-dev team and other iOS hackers around the world. Now we can fix SIM not supported message in iPhone 3G and 3GS after iOS 4.2.1 firmware upgrade too.