How can I get my SIM card unlocked on my iPhone?

If you bought a used iPhone from somebody and have now discovered it is carrier-locked, contact the seller.

If the seller has disappeared, you’re going to need to discuss this with the carrier holding the lock.

Carriers may or may not be willing to unlock an iPhone for somebody not their (current or former) customer.

There is nothing Apple or anyone here can do to resolve the problem. You will need to contact your cell provider as they are the only ones that have the information that can help you.

Contact the network provider and ask them to unlock the SIM card.