IMEI number you entered doesn’t match an AT&T device

My brother-in-law has tried to unlock his old iPhone 6 that is off contract and paid off through his account, but it says that the IMEI is not recognized. I have tried the website services to unlock the phone and to no avail. Unfortunately @ATTHelp no longer receives private messages so a solution previously posted does not work. I keep getting an error that "IMEI number you entered doesn’t match an AT&T device". But one of the online services I used, has confirmed that is on the ATT USA carrier. Is there a way to have request made for non-ATT clients with unrecognized IMEI numbers to get their Phone unlocked.

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