Trade in issues

Hello - I am hoping someone else has encountered my particular situation and can advise on what I should do. I purchased a brand new phone. When I received the new phone, I used the return shipping label included in my new phone to return my old phone for trade in. I thought this was the label I was supposed to use. I have contacted AT&T several times and spoken to Mobility Specialists who mentioned to me that this happens all the time.

No one seems to know where it went - even thought the shipping label was provided by AT&T. If it happens "all the time" it seems that AT&T would know how to handle this situation. 

My old phone that I traded in was in excellent condition and I have no idea what warehouse it went to. Please let me know what I can do to find my old phone.  This has been a nightmare. Please help!!! Thank you so much!!!!