The massive exodus comes after the country's second-largest mobile carrier began waiving early termination fees following a major data breach detected in September last year.
Of some 154,851 users who left KT from Dec. 31 to early this week, 15,701 or 64.7 percent switched to SK Telecom. Another 5,027 joined LG Uplus, while 3,524 moved to budget providers.
As part of its compensation package following the breach, KT is allowing subscribers to terminate their contracts without penalties until next Tuesday.
Rival SK Telecom offered a similar program after a major data leak last summer, which led about 166,000 subscribers to leave in about 10 days.
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