In my case the phone number was easier to keep alive than the banking side, mostly because topping up or staying on a low-cost plan was simpler than guessing when a bank would suddenly want fresh residence status. In Korea a lot depends on visa timing and the exact bank, but I would not assume dormant means safe. If someone is leaving for more than a short trip, I would check both the carrier’s minimum activity rule and whether the bank account is still usable for OTP or identity verification while abroad.