As someone who works in the cell phone industry in the USA, I can say with 100% certainty that iPhones are not already unlocked unless you purchased it directly from the Apple store. If you bought it from one of the wireless carriers (AT&T, etc) it will be locked to them unless you have it unlocked. Rule of thumb is, if you received it at any sort of discount (meaning you paid less than $500 for it) you will need to unlock it for international use.