I'm not going to panic as this has happened at multiple retailers, online, through email, phone scams, etc. My mother used my identity when I was 18, the key is to keep your eye on it. Contact Experian and get them to monitor it, I've been told it's free for 2 years. Don't know if that's for all T-Mobile customers or only the ones affected though.
Getting upset and yelling at T-Mobile won't help, Experian is at fault here. Contact them and be proactive. It's horrible this happened but just make sure you contact the company at fault and make sure they do the right thing.
I've got Experian watching mine for free for 2 years, I've also got LifeLock. It's not guaranteed but I can only do what I can and keep my eye on it from here...