Visit this website: http://big5.chinese.tour2korea.com/04Bookings/MobilePhoneRental/phone_rental.asp?kosm=m4_4
It's an Korea official website. I rent a phone there on January 2005. I booked through this network and got the phone when I reached the Incheon Airport. Your mobile phone can receive incoming call as well. As far as I remember, it's not so expensive per minutes. But better check before you use the service.