If you want to avoid using data, you need to deactivate mobile data on your mobile phone. Once done, you can only make voice calls, send SMS and use applications that don't require an internet connection.
Turn mobile data on or off
Slide your finger down the display starting from the top edge of your mobile phone.
Tap ALL SETTINGS.
Return to the home screen
Tap the indicator next to "Data connection" to turn the function on or off.
Tap the home icon to return to the home screen.
*Please refer to the Microsoft website for more information or further support.*