Definition of “Unlocked iPhone”

So you may be wondering, “Will my sim card work in one of your unlocked iPhones?”.

An unlocked iPhone will work for any carrier that
1) Uses a sim card
2) Works with GSM technology

The first part is easy, the second part maybe not so much. There are a few ways you can figure this out. The best and most accurate way is to just call your carrier and ask them. If for some reason that is not an option you can always just Google your carrier and “GSM”. Example: “Is T-mobile a GSM network?”. The answer, yes. 75% of carriers in the world are GSM. AT&T is included, so AT&T users have the choice of buying an iPhone that is unlocked or one that will only work with AT&T.
Here is a list of carriers in the US. Just look for GSM in the second column where is says “Voice Technology”.