Don't fear, a solution is here!
I went through this issue three times, yes three times they reported me not returning the phone. If you see a charge on your bill, call them up and they will open an internal investigation. Verizon has all the tracking numbers on file, even the one from your house back to the shipping center. The agent can pull up that tracking number and plug it in to fedex to verify it was delivered. If that is the case, they go search the warehouse for the phone, and usually give you a call the next day saying they found it.
The problem is, is that if the invoice has already been generated, they won't remove the charge from your bill, which means, my family ended up paying an $800 bill. Instead they just use the extra amount you paid and credit you for it on your next bills.
If you have any questions let me know.