AT&T claims never to have received phone I mailed back to them 4 months after the fact.

I upgraded my phone sent the old phone back in box and with return preprinted label.  4 months later my phone bill has a charge of 1300.00 added to it due to no record of old phone being returned. I have been on the phone with AT&T at least 6 times all 30 plus minutes of wait time. I've gone back to the UPS store I dropped it off at. I'm at a total loss and ended up paying the bill today to avoid my phone service being disconnected tomorrow. Why did they wait 4 months before telling me they didn't receive the phone? I'm certain I took a picture of the box with the label on it but I deleted it after a few months. It looks like I'm not the only person who has experienced this. Should I file a complaint with the BBB? I'm planning on continuing to pursue the issue. I just can not imagine AT&T not having records of their return labels.