If you want to use Wi-Fi without being charged for carrier data follow the guide:
1. you have to keep 'DATA SERVICES' (Networks and Connections-> Mobile Network) ON.
(the same with 'WHILE ROAMING' - ON, don't know if this one is obligatory,
but better to have it this way)
Otherwise, your wifi wont work.
2. But then BB use sometimes also data from your operator, that are charged.
TO AVOID this just go to SERVICE BOOK (Options->Device->Advanced System Settings->TCP IP) and delete all the lines starting with WAP, WAP2... (I kept only: 'Eurotel MMS Transport [WAP]'', Wi-Fi TCP/IP Browser[BrowserConfig], Wi-Fi TCP/IP Transport[WPTCP], BlackBerry Identity[BBID])
(for those who dont know how to delete lines: press BB key, then you will see Delete option)
If you will need use operators data service, keep configuration sms, and you can use it in emergency situation, where no wifi is present, to restore all deleted lines required,
Thats it

GOOD LUCK
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