AT&T has confirmed an April breach in which the personal information of an unknown number of users was improperly accessed.

"We recently learned that three employees of one of our vendors accessed some AT&T customer accounts without proper authorization," AT&T said in a statement. "This is completely counter to the way we require our vendors to conduct business."

"We know our customers count on us and those who support our business to act with integrity and trust, and we take that very seriously," AT&T continued. "We have taken steps to help prevent this from happening again, notified affected customers, and reported this matter to law enforcement."

Social Security numbers and call records were accessed between April 9 and 21, according to CNET. AT&T did not say how many customers were affected, but CNET pointed out that California law requires the disclosure of incidents that affect at least 500 local customers.

The data was reportedly breached in order to reveal the request codes that can unlock AT&T phones. AT&T currently will unlock a device for any customer whose account has been active for at least 60 days, whose account is in good standing and has no unpaid balance, and who has fulfilled his or her service agreement commitment.

Late last year, AT&T joined the nation's top wireless carriers—Sprint, T-Mobile, U.S. Cellular, and Verizon Wireless—in an effort to make it easier for consumers to unlock their devices.

The companies committed to a number of services, including posting information about unlocking policies, unlocking the phones of customers who have satisfied their contract, notifying users when their phone is eligible for unlocking, and unlocking devices for deployed military personnel.