Am i being charged for using 3G in iPhone?

I just recently got an iphone4 on ebay which was locked to AT&T but I got it unlocked so I can use it on tmobile. When I leave a Wi-Fi area it automatically says 3G in the top left corner. I don't have a data plan yet I can still use the Internet when it says 3G. I wanted to know if I was being charged without knowing?

Yeah there's a option in setting that's enable you to use 3G all you gotta do is go in the setting networks and disable the 3G network and that should do it i had the same problem and I also have iphone 4

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