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I am about to travel overseas (USA) for a period of time and looking to use my iPhone with a sim card from there to avoid high bills in relation to roaming and the like. I purchased my IPhone at an apple store, but not sure if it is locked to my provider, how do I work out if it is locked?
Try a SIM from a different carrier.
Frankly, if you bought the device in the US it is very likely locked to AT&T. The only way to get an unlocked iPhone in the US is to buy directly from Apple and specify unlocked which would be at full retail price.

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