Device Unlock Error - AT&T says IMEI number doesn't match a compatible device
I can't unlock my iPhone 6 (which is out of contract) because AT&T claim the IMEI number dosn't match a compatible device. This is the error message that I have been getting 3 times in a row:
"Your unlock request was denied because the IMEI number doesn’t match a device that’s compatible with the AT&T network.
Check the IMEI number by dialing *#06#, or check the device settings, and then submit a new request."
This is of course nonsense, since I have been using my iPhone on the AT&T network for 2 years. I triple checked the IMEI number and it is correct. The device gives me the same correct number either when I look it up through device settings or when I dial *#06#. This iPhone - purchased through Apple and activated through AT&T - was on a 2-year plan and is now paid off.
I really need to unlock this phone. Customer service can't or won't help. Why can't this issue not be fixed by a simple phone call?
Thanks in advance for any help.