Employee lost my trade in phone

I purchased a new phone in October and used the trade in program at the 3979 Wasatch Blvd Suite A Location. As instructed by Devin, an employee at the store, I left my phone with him and asked if there was anything further I would need to do. He let me know that was all I needed to do, and that he made a note on my account indicating my old phone had been traded in.

My last two bills have not reflected this credit, so I spoke with someone from customer service yesterday, who said they have no record of my old phone being turned in. They said I would need to call the store to find out what happened, which I did. The store told me I would need to track down Devin because he no longer works at that location and that they couldn't help me. So not only is it somehow now my responsibility to chase down wherever Devin might be, what if he happens to leave the company, are you honestly saying he is my only shot at figuring out where my phone ended up? This seems insane to me. 

I now have no way of finding out where my old phone is, and no one in the company seems to be able to help me. I am furious at the awful customer service I have received so far, and for all I know, my old phone was stolen by your employee. 

Can anyone in this company do the right thing and help me?