One of the very important things you should know about an iPhone is its Lock Status. The iPhone Lock Status will determine whether your device can operate with your local Carriers.
This is the reason that an iPhone’s Lock status should be UNLOCKED, unless you want to use that iPhone only with that specific Lock Carrier/Network.
There are a few ways to check an iPhone’s Lock Status, which should be the first thing to do BEFORE buying or starting to use that iPhone. Read more