KT said the online hackers -- formally caught on Weekend -- had thieved details such as clients' names, telephone figures and residential registration figures for five months since February and sold the details to telephone selling firms.
"The number of affected people account for nearly a half of about 17 Million clients of ours," a KT spokesman told AFP, adding the organization had alerted law enforcement on September 13 after detecting traces of coughing strikes.
Yonhap news agency, citing law enforcement, said the duo -- along with a former veteran programmer at a local IT organization -- had earned at least 1 billion won (about $880,000) by selling the thieved details.
Seven other people were also booked for buying the published details for telephone selling purposes, Yonhap said.
"We deeply bow our head in apology for having your precious individual details published... we'll try our best to make such things never happen again," KT said in a statement to clients.
Hacking strikes on significant organizations aimed to get entry to the individual details of their clients is a frequent occurence in Southern region Korea, one of the world's most-wired nations.
Seoul government bodies said in September last year online hackers using an Internet address registered in China had gained entry to Southern region Japanese significant websites such as web portal Nate.com and may have thieved the details of 35 Million customers.
In November 2011, Seoul's top games developer Nexon saw individual details of 13 Million customers of its popular activity MapleStory thieved by online hackers.
In March 2010, government bodies launched a probe into the security systems of significant retailer Shinsegae and 24 other organizations after details on 20 Million clients was published.
