Will updated iPhone 4 work for all AT&T users?

I was using an AT&T iPhone 4 for a while and then got a new phone because my contract ended so I went into iTunes and reset the iPhone to get rid of all my apps and stuff but the factory reset also updated it to the newest iOS (6.1.3 or something like that) and baseband. If I give or sell my phone to another AT&T user will they be able to use it? Granted they have the micro SIM card.

Yes, they can activate it at AT T, it is a fully functional phone. They will not be able to jailbreak it until ios 7 but for most thats not an issue.

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