Universal IPhone

There is no "universal" version of iPhones. Phones sold for different regions have different LTE bands. With the X series the differences are minor, and probably won't be noticeable. Worst case you may have only 3G in some areas, but the phone will still work. In addition, there are local feature restrictions on phones sold in some markets, specifically China, Japan, India and some Middle Eastern countries.

The bigger question is your phone SIM-unlocked? The only place you can buy an unlocked phone is directly from Apple, either online or in a store, and you must explicitly ask for an unlocked phone or you won't get one. If the phone is locked to a carrier you must check with the carrier to see what their terms are for unlocking it.

You'd have to call ahead to see if your model phone is supported on the local cellular network.

A Best Buy phone cannot be used with a SIM from outside of the US, PERIOD. and it will lock to the first carrier it is used with. That’s what they meant by “universal”. It will still work as a roaming phone with a US carrier, but that is likely to be expensive. You can buy from Apple.com online, and there is an Apple store in Syracuse.