There are 15 million T-Mobile US customers and applicants stolen personal information by hackers.
The breach was at a unit of the credit agency Experian, which T-Mobile uses to process information on subscribers.
Chief executive John Legere said he is “very angry”.
Names, birth dates and social security numbers are among data stolen, but not financial details, the firms said.
Chief executive John Legere said his company would review its link with Experian. “Obviously I am incredibly angry about this data breach” he said.
Subscribers who were credit-checked between 1 September 2013 and 16 September 2015 are most at risk.
In a statement, Mr Legere said: “I take our customer and prospective customer privacy very seriously. This is no small issue for us.
“I do want to assure our customers that neither T-Mobile’s systems nor network were part of this intrusion and this did not involve any payment card numbers or bank account information.
“Experian has assured us that they have taken aggressive steps to improve the protection of their system and of our data.”
Experian North America chief executive Craig Boundy said in his own statement, “We sincerely apologise for the concern and stress that this event may cause.”
It was unclear when the breach was discovered, but Experian said the matter was reported to the authorities immediately it learned of the hack.
The company said in a statement: “We continue to investigate the theft, closely monitor our systems, and work with domestic and international law enforcement. Investigation of the incident is ongoing.
“Experian is notifying the individuals who may have been affected and is offering free credit monitoring and identity resolution services for two years. In addition, government agencies are being notified as required by law.”
The firm said there “is no evidence that the data has been used inappropriately”.
There have been a string of high-profile hacks of businesses and other organisations in recent years affecting millions of people, including adultery website Ashley Madison, Sony Pictures, and retailers such as Home Depot, Target, and eBay.
T-Mobile is now the third biggest mobile firm in the US, having surpassed Sprint this year.
(Followed by BBC)