As long as you are inside your wireless provider's coverage area and your cellular account is in good standing, your iPhone should connect to your carrier's cellular network. If there isn't a problem with your carrier's network, troubleshooting your iPhone's settings, wireless connection features and the SIM card can help to determine the cause of the problem.
Enable Cellular Data
Activate the Cellular Data feature to see if your iPhone can connect to the cellular network. When your iPhone has a cellular data connection, the corresponding cellular data icon appears in the status bar, such as 3G.
Tap the Settings icon on the device's Home screen to open the Settings menu, select Cellular and then slide the Cellular Data switch to the On position.
Check the SIM Card
Open your iPhone's SIM card tray by pushing the SIM eject tool into the tray's hole. Remove the SIM card from the tray and inspect it for damage. If there is any visible damage, such as a crack, your iPhone may not be able to connect to the cellular network. Ask your wireless service provider for a replacement card.
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Disable the Airplane Mode Feature
Open the Settings menu and then slide the Airplane Mode switch to the left to disable the feature.
Swipe a finger up from the bottom of your iPhone's screen to open the Control Center. The Airplane Mode icon is highlighted if the feature is active. Tap the Airplane Mode icon to disable the feature.
Reset Network Settings
Resetting your iPhone's network settings clears all of the stored cellular network data, which may be preventing the device from connecting to the network.
Open the Settings menu and then tap General.
Tap Reset at the bottom of the screen to open the Reset screen.
Select Reset Network Settings from the list of Reset settings.
Enter your iPhone's pass code if you have activated a pass code for your device.
Tap Reset Network Settings from the pop-up menu to return the network settings to factory defaults.
Check for a Software Update
Apple periodically produces iOS updates that are available for installation from the Software Update section of the Settings menu. Updating iOS can rid any bugs that may be causing a cellular connection failure.
Open the Settings menu, tap General and then select Software Update. Tap Download and Install to install the update.