Being charged for phone after returning back to Assurant...

I am experiencing the same exact problem. I shipped out my defective phone almost 3 months ago and now am being billed $311 because they haven't received it. Why 3 months later? T-mobile says nothing can be done, Assurant says nothing can be done - both pointing fingers at each other. In the meantime, I am stuck with an astronomical bill and no resolution in sight. I have opened a case with the USPS (let's be honest, slim to no chance that will do any good) and have no idea what else to do. I refuse to pay for something that wasnt my fault - $200 for deductible and now $311 for the defective device never being received? If this cannot be resolved, I will be leaving T-Mobile, who by the way, has had the worst service in the Miami area since Hurricane Irma (phones stopped working 2 days before the storm hit!). AT&T and Verizon customers in the area never had as much as a hiccup in their service. T-Mobile is striking out with me, big time.