The company says the alleged hackers stole subscribers' names, phone and personal identification numbers and then sold the data to telemarketers, making an estimated $877,000, the BBC reported Monday.
An illegally installed computer program in the company's network had collected the information over several months, KT Corp. said.
"It took nearly seven months to develop the hacking program and [the suspects] had very sophisticated hacking skills," the company told the Yonhap news agency.
KT, which is also the second-biggest mobile operator in South Korea, apologized to its subscribers, saying it would "strengthen the internal security system and raise awareness of security among all employees to prevent causing inconvenience to customers."
Hacking attacks are common in South Korea, where Web sites and portals, a government-backed bank, government ministries and one of the country's largest games developers have been hacked in the past year, officials said.
Caroline Berg Eriksen: Soccer player's wife triggers debate with post-birth selfie
Britney Spears on kissing Ryan Gosling, Justin Timberlake in the Mickey Mouse Club