T-Mobile said a "bad actor" stole data on about 37 million customers after first gaining access to a company system in November, according to a Securities and Exchange Commission (SEC) report filed on Thursday.

The company said the hacker stole customer data that included names, billing addresses, emails, phone numbers, dates of birth, T-Mobile account numbers, and information describing their service with the wireless carrier.

T-Mobile's ongoing investigation concluded the data breached did not include passwords or PINs, bank account or credit card information, Social Security numbers, or other government IDs.

"Our investigation is still ongoing, but the malicious activity appears to be fully contained at this time," T-Mobile said, with no evidence the intruder was able to breach the company's wider network. It said the data was first accessed on or around Nov. 25.

"Our systems and policies prevented the most sensitive types of customer information from being accessed, and as a result, based on our investigation to date, customer accounts and finances were not put at risk directly by this event," it added in the filing.

Nevertheless, stolen information can be compiled with other stolen or publicly available data and used by scammers to steal people's identities or money. T-Mobile said it is working with law enforcement and has begun to notify customers whose information may have been breached.

T-Mobile noted that it began a "substantial multi-year investment" to improve its cybersecurity in 2021.

"Protecting our customers' data remains a top priority," the company said. "We will continue to make substantial investments to strengthen our cybersecurity program."

The company has been hacked multiple times in recent years. In July 2022, T-Mobile agreed to pay $350 million to customers who filed a class action lawsuit after the company disclosed in August 2021 that personal data including Social Security numbers and driver's license info had been stolen.

Nearly 80 million U.S. residents were affected by the breach.

After buying rival Sprint, T-Mobile became one of the country's largest cell phone service carriers in 2020. It reported having more than 102 million customers after the merger.

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