A properly functioning SIM card is essential for acquiring and maintaining cellular service on your smartphone. If you've recently installed a new SIM card in your iPhone and aren't able to get service, perform a series of troubleshooting procedures to diagnose and fix the problem.
Start with some basic troubleshooting. Make sure you're using a certified SIM card set up for use in your home country. Many SIM cards work only with domestic carriers and cannot connect to a network outside of the country for which they were designed to work.
Next, double check that your SIM card is inserted correctly. Make sure the card is inserted flat and that it's oriented according to the diagram located near the SIM card slot.
Restart the phone and wait for it to try to acquire service.
Verify Your Cellular Connection
Open the Settings app and tap Cellular.
Toggle the Cellular Data switch off and then on again to try and reestablish your connection to the network.
Tap Cellular Data Network and then touch Reset Settings.
Return to the main Settings screen, select Carrier and make sure the Automatic switch is set to the On position.
Reset Network Settings
Open the Settings app and touch General to view the General settings panel.
Tap Reset to open the Reset panel and then touch Reset Network Settings to reset the iPhone to its network settings defaults.
Install the Latest Updates
Connectivity issues can often be resolved by installing the latest patches and fixes from Apple and from your mobile service carrier. Both types of updates are available through the General section of the Settings app.
Before installing the latest software and firmware updates, make sure your iPhone is connected to a power supply so that its battery doesn't die, which can damage the phone.
Open the Settings app, tap General and then select About. If an update from your carrier is available, a notification appears on your screen prompting you to install it. Follow the prompts to install the update from your carrier.
Return to the General settings screen and this time select Software Update.
Tap Download and Install and then touch Install to initiate the update process. Depending on the speed of your iPhone and its Internet connection, this process can take more than half an hour.