You can cut data roaming costs and save on your phone bills by changing cellular data settings. To use the optimal connection method, change the Wi-Fi and cellular networks settings.
Data roaming means using your phone to receive data over networks that your network service provider doesn't own or operate. Connecting to the internet when roaming, especially when abroad, can raise data costs substantially.
Using a Wi-Fi connection is generally faster and less expensive than using a cellular data connection. If both Wi-Fi and cellular data connections are available, your phone uses the Wi-Fi connection.
Connect to a Wi-Fi network
Tap Settings > Wi-Fi.
Make sure Wi-Fi networking is switched to On.
Select the connection you want to use.
Close the cellular data connection
Swipe down from the top of the screen, tap network_cell and switch Cellular data to Off.
To follow your data usage, tap Settings > Data usage.
Stop data roaming
Tap Settings > More > Cellular networks, and switch Data roaming off.