Unlocking my iphone device

I submitted a request for AT&T to unlock my device and the response I get is sorry we can't unlock this AT&T Prepaid device right now - device has not been active for 6 months.

But I have been using the device as AT&T prepaid customer for more than 1 year and all the bills have been paid in full including a proof of purchase from Best Buy where I bought the full for full price for an Unlocked phone.

I need help in finding out where I can send my proof of purchase to confirm that the device is eligible for unlocking by AT&T