Check out Onesuite.com, they got
calling cards and
VoIP service.
Rate to US from S. Korea using Onesuite calling card service is 13.9 cents/minute (You dial an access number in Korea 00308131593 then punch in your PIN then preifx of 001 and finally the 10 digit US number.
Rate to US using its VoIP service calling from anywhere is just 2.5c/min. Just dialled the 10 digit US number directly.
HTH