zhaomingsu, I am so sorry to read about the way this played out. Not only does this experience sound terrible, but it seems like you put out an extraordinary amount of effort to find out what was happening here, and it took months to get an answer - I agree that is way too long.
Typically, when deferred (mail-in) trade-in equipment is evaluated and found defective in some way (if it has physical or liquid damage, doesn't power up, or still has FMiP/an activation lock enabled), the customer should actually be contacted by the vendor that evaluates the equipment and given the opportunity to get the phone back. It's a small window, but it's my understanding that multiple messages are sent, and a customer should be able to view this information on MyT-Mobile as well. If the ball was dropped on this matter, that's awful. I don't blame you one bit for your frustration, and I definitely don't want you to think that it's anyone's aim to take advantage here.
The Support Community is a user forum, so we don't have the ability to take a look at your account to determine what happened here with your trade-in, but we definitely want to ensure everything possible is done to get you accurate information. I know you haven't had the best luck getting a consistent answer when reaching out over the phone - are you comfortable interacting on social media? Our Contact Us link includes the information for our social media team, T-Force - you can reach out to them on Facebook or Twitter, and the contents of the thread are still there even if you switch representatives or communicate over multiple days, so you should be easily able to reference previous interactions. I'm going to send you a PM as well, which you can check here.
Thank you for taking the time to describe what's happened - please know that we will be passing along the feedback about your experience.
Im writing this here just tell every customers how Tmobile company cheating customers in this trade in promotion. Please tell the supervisor I am going to report your company and let the court deal with it； Thank you