Unlocked iPhone users unite

I got an unlocked iPhone 4 on eBay. Sealed, new and all. I plan on using it on T-Mobile, so it's a GSM/SIM phone. But something bothers me. Straight out of the box, it was ready to use. It was linked to AT&T with complete 3G access. I haven't opened the SIM tray yet, but there's already a GA phone number with it (912 area code). The seller's from SC. Is it supposed to be like this?

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