Experian was hacked recently and in the process confirmed today, 15 million t-mobile customers info has been compromised.
If you applied for t-mobile between September 1st 2013 to September 16th 2015, you may have been one of the victims. Experian has set up a page to tell details and also offer complimentary credit monitoring.
I dont even remember if I have applied to t-mobile service in the last 2 years.
Heres more info