This will work if you receive the following message in AT&T clean service:
"We can't unlock this device because of a past due or unpaid balance on the account"
"Looks like you have an unpaid or past due balance. to make a payment and then wait 24 hours before you submit a new request"
"We can't unlock this device because it hasn't been active for at least six months of paid service."
" Your device seems to be active on an AT&T customer’s account."
If phone is blacklisted (lost/stolen) will not work for sure also probably will not work if has unpaid bills or is fraud.