This is not looking good. T-Mobile is apparently investigating a possible breach that could involve the data of up to 50 million people. The news broke when Vice picked up that the hacker was trying to sell off a portion of the data in exchange for Bitcoins. The team at Vice have been able to validate that the sample data provided looks legit.
The data, reportedly stolen from multiple T-Mobile servers, contains user identifying information, which includes names, addresses, and phone numbers; social security numbers; IMEI numbers, all of which are unique to each mobile device.
You can read the story in full here