IPhone carriers - SIM Cards?

I'm looking to buy an iPhone, and the one I'm looking at doesn't have the carrier listed and the seller isn't replying.

So my question is, if the carrier isn't right for me, can I just take out and replace the SIM card with a card from my carrier? Or is it, like, set?

Is there a way to unlock the phone to a different carrier if it's not on the right one?

The carrier I'm going with is one that carries iPhones, so I just need to know that if the phone comes with a separate carrier, can I change that?

An iPhone usually locks to the first sim card put into it. So if this phone has had a sim card in already it most likely will not work with your carrier (unless it is the same)
Best to find out from the seller before buying to be on the safe side.

If you unlock the iPhone, you can choose your carrier. Of course there's a way… Millions have done just that.

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