Using the data connection network of your Android phone while roaming may result in additional charges. To deactivate and disable data roaming on HTC One do as follows:
1. Open "Settings" App
2. Tap "Mobile Data"
3. Make sure "Data Roaming" is unmarked (To enable data while roaming, mark the option)
4. You can also mark "Data Roaming Sound" to be notified when connecting to roaming network