@HyungJin If they call you depending on your plan it is free if you have unlimited minutes but if you have an allotted amount of minutes per month then it will use those minutes up but no extra charges.
For you to call them it will cost you:
South Korea is $1.09 per minute
North Korea is $3 per minute
If you will be calling alot then you can get a $5 International Preferred Rate Add-on then the cost will be:
South Korea $0.35 per minute
Sorry no preferred rate for North Korea
You can get all the info here.
