AT&T has warned customers of an attempt by hackers to access online account information.
"We recently detected an organized and systematic attempt to obtain information on a number of AT&T customer accounts, including yours," the company told clients.
"We do not believe that the perpetrators of this attack obtained access to your online account or any of the information contained in that account."
The unknown attackers appear to have used an automated script to try and link AT&T phone numbers to online login credentials, which could then have been used to access custoemr accounts.
While the company says that fewer than one percent of customers were affected, this could still amount to more than a million people.
AT&T's warning customers to be on the alert for phishing scams - although it claims that in telling customers about the attack it's acting out of 'an abundance of caution'.
The company says it's working to establish who was responsible.
The company was hit by another major attack last year, when hackers managed to snaffle more than 100,000 email addresses belonging to iPad 3G users. One of the two men accused of the hack pled guilty in court in June.
Yesterday, many AT&T customers on the northeastern US lost data service for three hours. AT&T says this was unconnected to the hacking attempt.