Sim carrier lock

If you go to Settings > General > About

There should be Carrier Lock and the carrier the phone is locked to, should appear there

That's who you need to contact to see if the phone qualifies to be unlocked and what, if any costs may be required to be paid.

You pay your carrier every month for service, so you would know who they are unless you are really telling us that you bought a phone from someone and don't who the carrier might be.

In that case, you will need to ask the person that you bought the phone from which carrier they were using and you will also need to get their cooperation to have the phone unlocked. For example, even if you find out which carrier the phone is locked to, the carrier will not unlock for you.

If the person that you bought the phone from can't help, return the phone for a refund. The phone was not likely theirs to sell and you don't want to be in possession of stolen property.

The reality is that since you’re presumably not a subscriber of the “Locking Carrier” …

… they have virtually zero incentive to unlock it for you.

And ONLY the Locking Carrier can remove the lock.

How to unlock your iPhone for use with a different carrier - Apple Support

Sorry.