Good morning pankhania. I'm so sorry to read about the trouble you're having! Billing concerns are terribly important regardless of the dollar amount, but waiting for a high dollar adjustment when you have multiple users on a family plan impacted is even worse. It sounds like you've got every right to be frustrated with us if we've promised call backs and have not delivered - I do want you to know that this is something we take very seriously. There should be system records of promised follow-ups on your account, and if those commitments are missed then I assure you that we hold ourselves accountable.
That said, since the Support Community is a user forum, moderators here don't have the ability to review individual account records, so regrettably we can't take a peek at this ourselves. We definitely need you to Contact Us so that we can remedy this situation. What I can do from here is clarify the process for resolution. With an insurance claim, the return label provided is directed back to the insurance agency, but with a warranty exchange (even extended warranty) the original or returned replacement device is sent back to T-Mobile. Did you pay a deductible (typically with the device you're describing it would be around $175.00) for this replacement?
When we investigate these returns it's imperative that we know whether we should be reviewing our own warehouse or working with the insurance company's records for the return. If you used their return label, then the equipment you returned was shipped to them and they may have a record of the tracking number they provided - if you used ours, then it would be shipped to us and a standard warranty record would include return tracking information (although more and more frequently this process is handled in store and returns are immediately shipped by our staff to prevent this type of issue). It's also important that the correct IMEI is being searched for - whether that is the serial number of the replacement provided or of your original device. Again, I wish that I were able to research this issue for you myself, but in lieu of that I want to do my best to identify possible gaps in the process that may be preventing a resolution, so that you have the best understanding of the situation when you reach out for help to a team that has account access.
Please let us know if you've been able to identify any possible issues with the research being done from the information I'm providing, and thank you for taking the time to let us know about the experience that you are having.