Being charged for a phone that was returned and apparently was stolen or fell out of the package

So my fiance is currently being charged for a phone that he returned through his insurance coverage plan with T mobile. He was told that the phone was sent back and the warehouse received the package but apparently the phone was not inside it. So either this phone fell out or it was stolen, which is outside of his control of course. His phone service has been cut off 4 separate times now due to this issue and he keeps having to call during business hours and get it turned back on, which is very inconvenient, especially since one of those times a family member went missing and he could not make any calls. Today, he was told by one of the t mobile assurant representatives that he would have to call the United States Postal service himself for an investigation. As we know, investigations take a long time and his phone service keeps getting cut off randomly. Also, I am unsure whether it costs him anything to do the investigation and if it does cost money we will not pursue this unless T mobile would reimburse him. So basically, he is being charged for a phone that he no longer has and has to call the United States postal service himself for something that is out of his control. Quite frankly, this is ridiculous. So I was wondering if anyone on here had a better solution. If this cannot be resolved any other way he will have switch carriers and we will not pay for the charge for the phone he returned. If we must return his current phone, we will and he will have to purchase a new one with a new service. Hopefully this issue can be resolved so we won’t have to resort to that.