We're sorry. We can't unlock this device.

I have 2 of the iPhone 5S, these were purchased 5 years ago and are paid off in full, these are not company phones and my account is current.  Neither phone was replaced due to warranty or insurance.

I would like to unlock the phone so I can use it overseas.  When I enter the required information for each phone I receive the reply "We're sorry. We can't unlock this device."