tmobile created return label to return 4g equipment, i doped at ups store and ups lost thenpackage

T-Mobile created a label to return 4g lte cell spot, I droped it at the ups store but ups lost the package. T-Mobile charged me for the equipment and I found out that the equipment is lost by ups and T-Mobile does it want to file a claim and they are saying it is my responsible to get the equipment to them. 
i spoke to ups and opened a claim. Now the claim is approved but since T-Mobile is the one who created return label they wanted T-Mobile to provide documentation for the value of lost equipment so that they will pay them. 
No one at T-Mobile is addressing this and I am not sure who to reach out now.