Bear in mind that the model doesn't tell you if you if the phone is unlocked or not. that's done on an individual basis by IMEI. You can get an A1533 iPhone that is locked to AT&T, or it could be a T-Mobile unlocked version.
Only way to know for sure is to find an unlocking service that has an IMEI stats checker (swiftunlocks has one, it's $0.99 per IMEI), or put another carrier's SIM in the phone and see what happens.