I was in South Korea last year. I was carrying my iPhone 3G that I purchased in Australia. Why don't phones purchased from other countries work in KoreaI understand that they are on a much advanced Mobile network similar to Japan. So, I ended up renting a simple mobile phone from the airport..
It didn't work because it wasn't whitelisted. Before Dec 2010 foreign handsets had to be approved via an expensive testing process. Now it's possible to register you handset online.
Over UMTS-3G.
They don't have a GSM (2G) side, AFAIK. (There might be some GSM-GPRS networks around, though.)
SK Telecom uses a CDMA2000 network, with support for 1X voice, EVDO 3G Rev A and UMTS 3G with HSPA.
Just as UMTS-3G was tacked onto older GSM-2G networks, in this case UMTS-3G was tacked onto an older CDMA2000 network.
Pretty much nailed it. There has never been a GSM network of any frequency band in South Korea. One of the reasons folks here had to put up with AMPS for another two years whilst everyone was getting GSM. KMT/Shinsegi now SK both started with IS95 CDMA at 900. LG and KTF rolled out later at 1900.
At some point SK and KT went with 3G UMTS on top. LG and I guess Verizon in the US didn't.
Apple's classification of the two iPhone models is misleading. The GSM model is really both a 3G UMTS and a 2G GSM device. They are totally different technologies. So your "GSM model" works because it's a 3G model too.
(KT=KTF=olleh)