Xfinity Lost My Trade In

I've been a customer with Xfinity for almost 20 years. Back in March, my son and I decided to switch his service over to my account.  My son had a very expensive iphone that he had finished paying for and was told he could trade it in for a brand new phone and get 800.00 in credits towards a new phone that would give him a very reasonable monthly bill. We decided to go ahead and trade in his phone.  They gave us the shipping instructions and we sent the phone in. We kept checking the progress and the first bill came with no credit. We reached out to xfinity and they claimed we never sent the phone in...oh but we did. We had the tracking information from the post office to prove it.  It was delivered and signed for by xfinity days after we sent it in on April 3.  Since then I have gotten nothing but the run around. Xfinity says we need to call Assurant, Assurant says they can't help us and we need to call Xfinity, Xfinity sends us back to Assurant, Assurant sends us back to Xfinity.  Xfinity says they did receive the phone but it hasn't been updated in the system, then they say they haven't received in the warehouse...of course this is all after I have given them all the tracking information including the ID for the person who actually scanned the phone in at the warehouse in York, PA. It's been over 2 months of this and I'm at my wits end. We sent in a very expensive phone, that was paid for by us, in good faith and now they have lost our phone and we are stuck paying a high payment when we shouldn't have to.  My son worked hard to pay for that phone.  We could have sold it out right, but instead we trusted xfinity. I'm pretty sure we can kiss the phone and our lower bill good bye. I am so angry!  I'd switch to a different carrier but I am still paying on my phone through xfinity and can't afford to pay off the bill!  Someone at their warehouse probably stole the phone and we are footing the bill!!  Unacceptable Xfinity!  Unacceptable.