If you travel outside your carrier’s network area, you can avoid roaming charges by turning off voice and data roaming services in Settings > Cellular. 

Some airlines let you keep your iPhone turned on if you switch to Airplane Mode. You can’t make calls or use Bluetooth, but you can listen to music, play games, watch videos, or use other apps that don’t require network or phone connections. If the airline allows it, you can turn Wi-Fi or Bluetooth back on to enable those services, even while in Airplane Mode.

Turn on Airplane Mode. Swipe up from the bottom edge of the screen to open Control Center, then tap the Airplan Mode button. You can also turn Airplane Mode on or off in Settings. When Airplane Mode is on, the Airplane Mode icon appears in the status bar at the top of the screen.

You can also turn Wi-Fi and Bluetoot on or off in Control Center.