The Apple iPhone automatically detects the correct carrier settings from the Subscriber Identity Module card you installed in the phone when you first set it up. The only time you might need to change the iPhone's carrier settings is if you travel to a foreign country and want to use the phone there. You can access and change the iPhone's carrier settings from the device's "Settings" menu.
Tap the "Settings" application icon on the iPhone's display screen.
Tap "Carrier" from the "Settings" menu.
Wait for the iPhone to detect and display a list of locally available carrier networks. Your current network displays in blue type with a check mark beside it. If the iPhone is set to automatically connect to the correct network, the "Automatic" label at the top of the page displays in blue with a check mark beside it.
Tap the name of the carrier network you want to use from the list of available carriers. The network name changes to blue type with a check mark beside it to show that the iPhone has connected to a different network.
Select the “Automatic” option to ensure that the iPhone always connects to the correct carrier network. If using the phone abroad, you might need to call your carrier to enable international roaming.
Switching to another carrier network at home or abroad can result in additional call and data charges. If you change carrier settings while in a foreign country, remember to switch back to the "Automatic" setting or your home network when you return.