Network unlock

Click on the link to read Apple's support article on how to unlock your iPhone for use with a different carrier. - How to unlock your iPhone for use with a different carrier - Apple Support - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.

Click on this link for iPhone: Wireless carrier support and features in the USA and Canada - Wireless carrier support and features for iPhone in the United States and Canada - Apple Support - including a list of which ones offer unlocking.

The phone is locked to a carrier. What is the name of the carrier? If you are not sure, the carrier is the company that you pay each month for cell service.

Whoever that might be, you need to contact them to get them to unlock the phone. Apple cannot do this for you.