How to make sure iPhone 2G on 3.0 used on ATT doesn't get charged with data?

[vader1990]

So I have an iPhone 2G on JB 3.0. I wanna make sure that when I use it on ATT it doesn't get charged with a data plan. I'm on a family plan where I got a phone from ATT [some crappy free phone] and told them to disable ALL data from it. Like do a Wix block or something. Now, I'm going to simply swap the sims from the free phone I got from ATT and this iPhone.

What else do I need to do to make sure I don't get charged for data? I have Wi-Fi literally everywhere around me, so the data plan is basically useless.

Thanks for the help guys!