The best part you can do is to change the phone to new provider and cancel the T-mobile service. If you get a collection letter, you can copy your bill and send it to credit reporting companies. They will remove your name from report.

This is the best option you can have since I have seen same situation with AT&T and one of my friend did the same. $1500 is a big time for anyone who even makes more then a million.

Good Luck.