Buy a CSL IDD card with roaming service + bring a 3G phone. According to the instruction of the card, it is ok. Can be used in Korea and Japan.
Last month l tried this in Japan (the charge is cheaper then using roaming service of my HK phone), but l couldnt make call because there was no network. Therefore l used my HK phone, again no network at the place l used it. Then l made call in some open area e.g. near building exit, and it's ok. But then l forgot to change it back to the CSL IDD card. Therefore l am not sure whether it works. May be it does. Otherwise why the menu said so.
Agreed with the above that you may buy an international calling card from the convenient store and use it. But the operation menu is not easy to understand + there are limitation in using those cards. It may take a while for you to understand it.
How about buying an IDD card and try to use it in Korea. If you cant use it in Korea, buy a tel card there. You wont waste your HK tel card because when you come back to HK, you may still use it. But you could not use the Korean tel card.