Here is the tricky thing about Korea.
When I got here a year ago. I knew my Iphone4S was unlocked. It had not hit the streets here in Korea yet. The place where I get my info from originally told me my phone was not unlocked. But it was. But to make the phone work had to be put into their network (not just sim install) it had to be put into by serial #. Anyway, I took it to a different KT dealer (one on the Military base), and this is what is unique about Korea. They said, $35 to try, if not work, you are stil out $35.
Since then, I been talking to the original KT place for the entire year, I was looking at getting KT as my home internet provider if I stayed a 2nd year. Then I would have gotten Fiber to the house with blistering speeds. Anyway with the Iphone5 coming out I talked to him about it. Mostly when they would get, and how outside phones would work on it. He told me that 3G connection no problem. LTE is going to be the problem due to differences in networks and require a specially tooled Iphone5. A troop of mine just got here, she had a Samsong phone that was from ATT and was supposidly LTE capable. Again as I was told $35 and we can see, no refund. She ended up getting a Galaxy II something that was 3G.. because it was cheaper to get that phone vs the attempt on her old phone.
If it does work with LTE, please advise, so I can relay to the two KT shops here that I deal with as they deal with 100's if not 1000's of American customers who have various phones from states. I am really curious if the LTE Works.
As for your statement about the International Iphone5 and the Verizo Ipone5. I saw a post somewhere showing the various frequences in the phone. The Verizon model had more than the International one. Both had some unique coverages that were not duplicated with each other but both had a majority. i woudl say they had 85-90 percent identicle frequences but there was 10-15 percent difference.
The key thing I can tell you is when you UNDO the process to leave Korea and go home, is to make sure you stop by the KT shop and have them remove it from their "database" network so it becomes "unlocked" again. If not that phone could become locked to their network. Not sure that makes sense but that was what was key in making it work here, is they put my phones serial in their database.