Resolution Advice Needed

I have been with tmobile since the Sprint merger. Prior to that I was with Sprint via nextel since early 2000's. I have had some good experiences but also some bad ones. Here more recently I have been having a hard time with customer service as it relates to a device replacement. After being out of the country with limited phone device use back in July and trouble shooting with techs upon my return in late July it was disclosed that my device had a update network issue and would need to be replaced with a new furnished device. When I recieved this replacemeny device there was no return label in the box. Nor was I provided return instructions prior. I recently recieved a bill for a "un-returned device" 1400 bucks. The first communication about this un-returned  device was initiated by me calling to learn what this exorbitant cost was related to. Only to learn that it was for the damaged device. After repeated explanation that I was unaware and have no problem returning the device as it is not in use as it does not have network service, I was instructed the window has passed and we cannot help. After asking for this issue to be escalated I have been promised to call backs that have not occurred. I'm consistently having to call back and follow-up about the issue.Only to be promised another call back and advised that there will likely be no resolution to remove the cost of the device. When asked for executive relations contact I am told there is no one else I can talk to but I can write corporate.  This is a very befuddling experience. All I am looking for is  reasonable understanding that there was a lack of communication via customer service and ignorance of knowledge on my part.  ¿Qué puedo hacer?