USA based AT&T says that some of its customer's personal details have been leaked to an outside agency.
In a letter that is being sent to the affected customers, AT&T says that employees at on of its service providers accessed account information as part of an effort to request codes from AT&T than are used to unlock AT&T mobile phones in the secondary mobile phone market.
The number of affected customers is also unknown, although the company has filed a notice with Californian regulators, which is only needed when more than 500 customers are affected within that State.
The breach took place between the 9th and 21st April this year.
"We recently learned that three employees of one of our vendors accessed some AT&T customer accounts without proper authorization," the company said in a statement.
"We have taken steps to help prevent this from happening again, we are notifying affected customers, and we have reported this matter to law enforcement," it added.
AT&T is offering a year's worth of free credit reporting to the affected customers, as the data breach could have included the customer's date of birth and social security number -- both of which are useful to fraudsters.
The vendor is currently unnamed.