Getting an iPhone 5 soon, but it's an import. What will I need to do with it?

I'm going on a trip to the United Kingdom soon to see my sister, who has offered to give me her old iPhone 5 to take back with me to the United States. I accepted the offer, but I want to know for sure if it'll work on the AT&T network. I believe the iPhone 5 that's waiting for me was originally on the O2-UK network, and that all of the UK's mobile networks are GSM. AT&T is a GSM network, so I can only assume that taking my AT&T SIM card and putting it in the UK bought iPhone 5 shouldn't be a problem, but is that true? And what else would I need to do? Do I have to contact O2 and/or AT&T? Does the device need to go through Customs for either country? Will I need to have it unlocked? Will it work with AT&T's LTE band? Will I ever stop asking all these questions about it? Okay, that's probably a "yes" on that last question, but I still want answers to my other questions. Any true help is much appreciated.

Go to the source… Call ATT… Call the ATT store near you…

The phone would need to be unlocked in order for any other carrier's SIM to work in it. As it stands now it would only work with O2 SIM cards.

No, customs is not an issue. Not sure of the LTE bands. Would have to check what differences there are between O2 & AT&T & what the phone itself uses, but it is pretty likely it wouldn't be a problem since the iPhones support the most data bands.

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