Unicom card can't receive verification code in Korea

Hello, I would like to ask why China Unicom can't receive the verification code in Korea. Unicom card can't receive verification code in Korea because the mobile phone is in arrears. Because there is no geographical restriction for Unicom card to receive verification code, it will be unable to receive verification code normally because of the arrears of mobile phone, which belongs to the phenomenon of arrears. Therefore, Unicom card can't receive verification code in Korea because the mobile phone is in arrears.