Log on to your online account and look at the copy of the bill. It should have the payoff amount on the bill.
Don't expect the phone to be unlocked immediately. It takes a week or 2 after you pay off the phone until the servers are updated to allow the unlock.
This is a user to user forum. I am a user, just like almost everyone else on the site.
Award for Community Excellence 2019 Achiever* *I am not an AT&T employee, and the views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.