The firm says the two stole subscribers' names, phone numbers and residential registration numbers, and then sold the data to telemarketers.
News agency Yonhap News said it was estimated the hackers made about 1bn won (£560,518; $877,000) from the sale.
KT is also second-biggest mobile operator in South Korea.
The company said the duo had installed a programme to collect subscribers' information over several months.
"It took nearly seven months to develop the hacking program and (the suspects) had very sophisticated hacking skills," the firm told Yonhap News.
"In light of this incident, we will strengthen the internal security system and raise awareness of security among all employees to prevent causing inconvenience to customers."
The company apologised to its subscribers, pledging to "strengthen the internal security system and raise awareness of security among all employees to prevent causing inconvenience to customers," Yonhap News quoted KT as saying.
In July 2011, South Korea blamed Chinese hackers for stealing data from 35 million accounts of Cyworld website and the Nate web portal, both run by SK Communications.
Seoul claimed that it managed to trace the source of the incursion back to computer IP addresses based in China.
Earlier, in April 2011, hackers targeted a government-backed bank in South Korea, and in May the same year, data on more than 1.8 million customers was stolen from Hyundai Capital.
Government ministries, the National Assembly, the country's military headquarters and networks of US Forces based in Korea were also hit in 2011.
In November 2011, one of South Korea's main games developers Nexon had experienced an attack as well, with personal details of 13 million users of its popular MapleStory online game stolen.
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