This setting appears on GSM networks when you’re outside 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 a roaming agreement
with your carrier. Additional fees may apply. Roaming charges may be billed to you by the other
carrier, through your carrier.
Select a carrier: Go to Settings > 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.