It has been revealed that hackers have stolen the personal information of 15 million T-Mobile US customers and applicants.
The breach was at a unit of credit agency Experian, which is used by T-Mobile to process information on its subscribers.
The firm said names, addresses and date of births were among the stolen data but no financial information.
John Legere, chief executive of T-Mobile, said his company would review its link with Experian.
He added: “Obviously I am incredibly angry about this data breach.
“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: “We sincerely apologise for the concern and stress that this event may cause.”
In a statement the company said: “We continue to investigate the theft, closely monitor our systems, and work with domestic and international law enforcement. Investigation of the incident is on-going.
“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.”