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
Return to the home screen
Slide your finger down the display starting from the top edge of your mobile phone.
Tap the settings icon.
Tap the indicator next to "Mobile data" to turn the function on or off.
Tap the Home key the necessary number of times to return to the home screen.
*Please refer to the HTC website for more information or further support.*