Two men suspected of accessing details illegally of 8.7 million users of South Korea's largest fixed-line phone operator, KT Corp, have been arrested.
The company says hackers stole subscribers' names, phone and personal identification numbers, and then sold the data to telemarketers.
The hackers made an estimated 1bn won (£560,518; $877,000) from the sale.
An illegally installed computer program had collected subscribers' 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 is also second biggest mobile operator in South Korea.
The company apologised to its subscribers, promising 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 said it had traced the theft of data from 35 million accounts from the Cyworld website and the Nate web portal, both run by SK Communications, to computer IP addresses based in China.
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 South Korean consumer finance company Hyundai Capital.
In November 2011, one of South Korea's main games developers, Nexon, was hacked, with personal details of 13 million users of its MapleStory online game stolen.
Government ministries, the National Assembly, the country's military headquarters and networks of US forces based in South Korea were also hit by hackers in 2011.