Device Unlock: We do not have a record of the IMEI you entered

Please help.  I just switched carriers and I am unable to submit a "Device Unlock" request to ATT using the website becasue of the IMEI error:  "We do not have a record of the IMEI you entered. Please verify the IMEI entered is correct by dialing *#06# on your device"

I called customer service and they said all i could do was enter it into the website, however, I keep getting the same message.  This is my son's phone who is away at college and I need to transfer this to the new carrier as soon as possible.

Can someone explain to me how I can get this resolved as soon as possible.  I had two phones to unlock.  One worked, and one keeps getting the IMEI error.

Thank you!