Another Korean here. Usually in Korea, one could refer to an unknown someone as "선생님(teacher)". "각하", on the other hand, has been referred to the President of Korea. So it really sounds weird to a Korean. When someone calls you "각하". If I were to be a customer, then I would be called "고객님". Where "고객" means customer and "님" is like "sama" in Japanese, and "sir" in English.