How to unlock a sim from a iPhone 7?

The carrier is the company that you pay your monthly cellular bill to. T-Mobile, AT&T, Verizon, etc. If you don't know who the carrier is, you don't own the phone. Get with the person that you got the phone from.

Call the carrier. Only the carrier can unlock the card.

Please note that they are under no obligation to do this.

You won't get the number back or messaging back until you get the SIM card unlocked.