I also have returned FIOS equipment and never got a refund credited to my last bill. I paid a security deposit on equipment several years ago and when returned at the close of my account, received no credit. I returned the equipment on 4/5/14 and have the receipt.
I did call customer service and a woman actually told me, "In my 16 years of working here I have never heard of a security deposit on equipment." Difficult to believe, naturally. Anyway, she promised to request my records, because the deposit was paid several years ago and she did not have access to those records and needed to request them from another department. She never called me back and I still am getting billed an outstanding balance, which should by now reflect the credit for returned equipment. Amazing how this policy is so important on the front end and then ignored on the back end.