Hi Min Jeong.
I am assuming that since you work and live in Korea that you always answer the phone in Korean first, however if you hear the caller speak English I believe a simple, "Hello, this is [...name...], how may I help you?" would be fine. Perhaps after that you could follow up with, "Is there a particular person that you need to speak with?" and then forward the phone call out.
Can you provide more information as to what a foreigner usually asks? Then maybe I can help form a sample conversation for you.
-Jo