T-Mobile said "the hacker acquired the records of approximately 15m people, including new applicants requiring a credit check for service or device financing from September 1 2013 through September 16 2015".
As much as this affects T-Mobile customers, not all records stolen were T-Mobile customers since this was a attack on Experian. The really terrible thing about this breach is that the hackers have enough data to file for a person's income tax return. Talk about a identity theft nightmare in the making.
However, you may still be protected. If you feel you've been affected by this data breach you can sign up for a complimentary membership to Experian's very own ProtectMyID Elite: http://www.protectmyid.com/
Take advantage of this free service. It will protect you from any identity theft attempts.
Learn more about this attack here: