Personal information belonging to 73 million current or past AT&T customers has been leaked online.
The dark web has released information such as addresses, social security numbers, and passcodes, according to the US telecoms company.
AT&T stated that it had not found evidence indicating that the data had been stolen, but had hired cybersecurity specialists to investigate.
Customers' passcodes have been automatically reset.
The company asked them to "remain vigilant by monitoring account activity and credit reports".
The data implicated in the incident appears to be from 2019 or before and is associated with 7.6 million consumers and 65.4 previous account holders.
It also includes full names, email addresses, and birth dates, though AT&T stated that no financial information was leaked.
The company said in a statement that it was unclear whether the data came from its own systems or a third-party provider.
AT&T's wireless 5G network serves around 290 million people in the United States, and the business is one of the country's leading mobile and internet service providers.
In February, a massive outage hit tens of thousands of phone customers, prompting the company to apologize and issue a $5 credit to those affected.
Prosecutors in New York initiated an investigation into the incident, which prevented people from using their phones for around 12 hours.