Support unlock US cellular galaxy s3

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http://androidforums.com – hi all =) i scored a GS3 for 50 bucks, but it is on US cellular which doesnt service my city. i got a microsim from my ATT carrier, but when i put it in it doesnt prompt me to enter an unlock code. I just did a factory reset on it and still nothing. how can i unlock it so that i can use my sim? (General)