From what I gather, AT&T doesn't lock a phone for missed payments…
I'm struggling to make sense of this statement.
If you bring an unlocked phone to a carrier then the carrier 'locking' the phone because of missed payments is not possible.
If you buy a phone from your carrier the device is locked to the carrier. Why would locking the phone due to missed payments change anything? It's already locked.
If you bring a phone that is locked to your carrier, to your carrier, why would this make any difference? Again, the phone is already locked.
If you buy a phone under Apple's upgrade policy the phone is unlocked. The carrier locking it due to missed payments is not possible (no idea if Apple would, but it would be possible for them to if they wanted).
If you buy a phone from Apple that is for your carrier and make payments to your carrier for the phone, the phone is locked to your carrier.
In short, a locked phone can't be locked as its already locked. And an unlocked phone cannot be locked or relocked because you can't do that to an unlocked phone.
Perhaps you meant something else by this? Blacklisted perhaps?