Charged 315 for non returned phone

There are several reasons AT&T might think it hasn't received a phone. The first step is figuring out exactly what the problem is that is causing the charge.  Why were you sending in the phone? Trade in, insurance, etc.? Have you checked the tracking to see if in fact it was received or if it is still showing in transit? 

@AlainaC21

Proof of return?

This would mean some kind of receipt either from the post office showing that you mailed the package. And tracking so that you can track your device through the post office and Pitney Bowes.  Or a receipt directly from a store who received your old device.

When returning the item it did not ask for proof of return so therefore i did not think to ask for a receipt. 

If you returned in the mail, your USPS Post Office receipt is proof of return. When returning at a physical store, you should have been provided with some kind of receipt. Go back to the store and see if they can reprint a receipt for you. Without either proof of drop off at the post office, or a receipt for turning in at a physical store you have no proof you turned in your phone and no way of getting that amount credited other than hoping that the device has just not been checked in yet.

It can take 2 months, or more, for devices to be checked back in. This time of the year there would be a very high volume of devices being turned in.