In a notice on its website, the telecom giant said a security incident occurred that saw "customer proprietary network information" (CPNI) improperly accessed. Some of the CPNI may include "phone number, number of lines subscribed to on your account and, in some cases, call-related information collected as part of the normal operation of your wireless service," T-Mobile said.
The company added the breach, reported to have impacted approximately 200,000 customers, did not include financial information, such as names on the account, credit card information, social security numbers, tax ID, passwords or PINs.
The company has experienced several other security breaches in recent years.
In 2018, a breach impacted the personal information of 2 million customers, with hackers gaining access to names, billing ZIP codes, email addresses, phone numbers and account data. In 2019, another 1 million prepaid customers had their information compromised by hackers.
In March 2020, the company suffered yet another breach, after hackers gained access to T-Mobile employee emails while also gaining access to certain financial customer data.
T-Mobile did not immediately respond to FOX Business' request for comment.Shares of T-Mobile were little changed in early trading on Monday.