How do i call someone who has a cell phone bought & setup in the u.s but is currently in south korea?

someone i speak to frequently use to live in south korea & has recently went back for a few weeks during the summer. she purchased & setup the phone in the u.s & took that phone with her to korea. i'm curious as to how i go about continuing to speak to her. do i have to dial any fancy numbers, etc ? will it cost more? i called the number a few minutes ago & it went to her voice mail which i expected because the phone is either dead or shut off. i'm on a family plan where my family has a set number of minutes per month & free night & weekend, plus i believe at&t began a new plan where at&t customers can call any mobile worldwide for free. i'm not sure about this whole thing as i have never been close to someone who has traveled this far so any help is awesome. thanks in advance