How to avoid AT&T Iphone Charges?

So i recently got a friend's Iphone 3g that I wanted to use my AT&T sim with. I turned off the data and 3g on it. In addition to that, I installed Cydia through the whole redsnow process. Is there any other necessary jailbreaking that I need to do? My friend said she did what I did (without the redsnow/cydia process) but eventually started getting charged. I don't want that to happen. Your help would be VERY appreciated!

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