This setting appears when you’re outside of your carrier’s network and other local carrier data networks are available to use for your phone calls, visual voicemail, and cellular network Internet connections. You can make calls only on carriers that have roaming agreements with your carrier. Additional fees may apply. Roaming charges may be billed to you by the carrier of the selected network, through your carrier.
For information about out-of-network coverage and how to enable roaming, contact your carrier or go to your carrier’s website.
Choose Carrier and select the network you want to use.
Once you select a network, iPhone uses only that network. If the network is unavailable, “No service” appears on the iPhone screen and you can’t make or receive calls or visual voicemail, or connect to the Internet via cellular data network. Set Network Settings to Automatic to have iPhone select a network for you.