What is a recommended first step when an iPhone displays "NO SIM"?

When an iPhone displays "NO SIM," turning Airplane Mode on and off is a recommended first step. This action effectively refreshes the iPhone's connection to the cellular network. Enabling Airplane Mode temporarily disconnects the device from all wireless signals, and turning it off again prompts the iPhone to search for available cellular networks anew. This can resolve minor connectivity issues that may cause the device not to recognize the SIM card.

Although other options, such as restarting the device, checking for software updates, or resetting network settings, can also be useful for troubleshooting, they may not address the immediate connectivity issue as efficiently as toggling Airplane Mode. Restarting the device might help reset the system, but it typically takes longer than simply toggling Airplane Mode. Checking for software updates is more relevant when compatibility issues arise, and resetting network settings is a more drastic measure that may be warranted later in the troubleshooting process if simpler methods fail.