I've never heard of ATT charging a fee to unlock a phone that's been paid for.
I have read where you have to do a backup and restore before a phone that was sold as locked (and then was later unlocked) before it will accept SIM cards from other carriers.
Here's a link to ATT's unlock request page.
https://www.att.com/deviceunlock/#/
I have read where you have to do a backup and restore before a phone that was sold as locked (and then was later unlocked) before it will accept SIM cards from other carriers.
Here's a link to ATT's unlock request page.
https://www.att.com/deviceunlock/#/