It will work on KT (they even sold the 3GS which wasn't CDMA), and possibly others.
He may need to get the IMEI registered with the network before the SIM will work in it though as they are very protective over their networks (this was true up until 2010 at least, although they may have become more relaxed on this since then).
I think I found the Answer I was looking for in How U.S. Carriers Fool You Into Thinking Your iPhone 4S Is Unlocked
Short answer "You can't do it!" If your phone is CDMA, its permanently locked to its CDMA network of origin.
Unlocking your Iphone 4S
Among the three major carriers operating in the U.S., only one, AT&T, will not unlock any iPhone. Because I purchased a Verizon-designated handset, the following information relates to those customers that purchase Verizon iPhones, but the technology and result is the same for any carrier.
When I called Verizon tech support to unlock my phone prior to leaving the U.S., I was told that I had to have a foreign SIM card installed in order to complete the process and that the phone would have to log in to the Verizon network on WiFi. The reason for this procedure is that the phone must validate the foreign SIM.
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So I stopped in at an Apple store in London. We logged into a WiFi network, installed a micro-SIM card from a carrier in England, and within about thirty seconds, the phone was unlocked because Verizon had already unlocked the phone at their end before I left the U.S. For the rest of my travels overseas, it worked flawlessly and provided essentially the same options that are available on any other dual-mode or GSM-only device.
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Four different iPhone 4S versions
When I returned to the U.S., I wanted to confirm that my Verizon iPhone could be ported to another carrier, whether CDMA or GSM. This was the intent of Apple when they designed the 4S: every phone is the same and will work on virtually any carrier in the world.
So, I contacted a regional GSM carrier in the Midwest that has roaming agreements with AT&T and T-Mobile. The carrier was kind enough to provide a micro-SIM card for testing in my iPhone. What I found may infuriate customers who have purchased expensive iPhones from one of the three carriers in the U.S. but at some point decide to switch to another provider because of poor coverage, customer service, or pricing.
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Sitting in South Dakota, I replaced my Verizon SIM with the Longlines Wireless chip. The phone indicated that the SIM was not recognized by the phone and required its removal and device reboot. I contacted Verizon global support to confirm my suspicions. They advised that indeed their phones were locked as against any other carrier in the United States, and that the term “unlock” only applied to overseas carriers.
I next inserted my UK SIM card into the phone, and it logged into AT&T without issue, which confirmed what Verizon told me. I then interviewed managers at three different Apple corporate stores to confirm issues with regard to unlocking.
What consumers need to understand is that there are actually four different versions of the iPhone 4S: Verizon, Sprint, AT&T, and Apple. Only the Apple phone, available from their stores or on-line, is fully unlocked and can be used on any carrier outside the United States. The other phones are permanently locked and cannot ever be used on another carrier in the U.S. Even if you spend $800 for an unlocked phone as I did and dedicate it to a single U.S. carrier, you are locked into that carrier forever if you want to keep using the iPhone. Neither Apple or the other carriers will fully unlock your carrier phone.
Best Answer
The only difference is cost.
Now that we've moved into a contract-free phone era, service providers, like Verizon, will offer to let you make a payment plan instead of paying for the phone upfront. In the end, the cost is the same as the SIM-Free version, but the Verizon version only works on Verizon until they unlock it.
Conclusion: Verizon iPhones are physically the same as SIM-Free iPhones, but Verizon has more control over the Verizon model.
Bonus note:
If you are considering getting an iPhone and are on AT&T or T-Mobile, the SIM-Free version will be a better deal because only the SIM-Free, Verizon, and Sprint models support CDMA, which is needed for those networks.