The cyber-security team found and shut down unauthorised access to certain information on Aug. 20, and reported the matter to authorities, T-Mobile said in a statement https://t-mo.co/2wrr9Cq on its website.
No financial data, social security numbers or passwords were compromised, but other personal information like name, email-id, phone number, zip code, account number or type (postpaid or prepaid) may have been exposed, the company said.
About 3 percent of T-mobile's 77 million customers could have been affected, a company spokesperson told http://bit.ly/2wknfLr technology news website Motherboard.
T-Mobile did not respond to Reuters request for comment outside regular business hours.
(Reporting by Ishita Chigilli Palli in Bengaluru; Editing by Sunil Nair)