Using Smart Dialing on your Blackberry might add a prefix to the international number you are trying to dial directly (without Voxofon). If so, the appended digits will make the number you are trying to dial incorrect; therefore, you need to disable Smart Dialing.
To disable Smart Dialing:
1. On the main screen, press the green 'Call' button
2. Press 'Menu' (or 'Options')
3. Scroll down to options
4. Select 'Smart Dialing'
5. Click on the top, where it says 'Country Code'
6. Scroll up to 'Unknown'
7. Press the 'Menu' (or 'Options') button again
8. Click 'Save'