I got my iPhone 4 in the US, and much to my chagrin I had to wait 90 days before I could use it in South Korea. It is AT&T policy.
For messaging, I use KakaoTalk 카카오톡. It is so simple and easy to use, and plenty of Koreans with iPhones have it so they should be able to add you easily.
The msn app for messaging is horrible. 네이트온 UC is ok.
Don't pay for your 문자!!!