1. Only your carrier can unlock your iPhone. Contact your carrier and request an unlock. Your account might need to meet requirements for unlocking. After you submit the request, it might take a few days to complete. Contact your carrier to check the status of your unlock request.
2. After your carrier confirms that they unlocked your iPhone, follow the steps below.

If you have a SIM card from a carrier other than your current carrier

Use these steps to finish unlocking your iPhone:

1. Remove your SIM card.
2. Insert the new SIM card. Your device will activate.

If you don't have another SIM card to use

Use these steps to finish unlocking your iPhone:

1. Back up your iPhone.
2. After you make a backup, erase your iPhone.
3. Restore your iPhone from the backup you just made.

1. Contact your provider and request an unlock using the contact details and online tools listed below.
2. If you haven't got the original SIM, you'll need to reset the phone before it can be unlocked.
3. Power down the phone, then replace the old SIM card with a SIM from a different network.
4. Check that the iPhone is able to connect a phone call over the new network.

That's the shortened version, but let's go through those steps in a bit more detail.