Issue with unlocking. Seeing "need to pay off installment plan", but fully paid off installment plan

I've been trying to unlock my iphone and have spoken to many different AT&T employees/customer service reps/advanced tech support about it, but have not received any answers or a solution. My iphone installment plan has been fully paid off and this has been confirmed by each person with AT&T that I've spoken to, but when trying to unlock my phone, it's showing the "need to pay off installment plan" error message. I was told yesterday, finally, that a case was being submitted for this request, since they couldn't figure out why I was receiving that message, and that I would get a follow up email with the case number. I have not received any email and another advanced tech support rep today said they didn't see a case submitted. My number was erroneously ported to a different SIM already, so I am currently without a working phone because of this mess.