Hello,
The only possible dialing error could be in the exit code you’ve been dialing (001). According to my sources South Korea has 2 such exit codes, the second being 002. I believe this depends on the carrier used to dial out. Try using 002 to see what happens. Or you can replace the exit code (whatever it is) with the plus (+) sign when calling from a cell. With many phones you achieve this by pressing * twice. Some may work in a different way.
If this still doesn’t work there could be a problem with the calling card you are using or your cell may be blocked for placing international calls. You can try using a different line/way to place this call.
If you still have questions please reply to this post.
Dan