South Korea and Japan have GSM 3G networks on 2100MHz BUT NOT GSM 2G networks.

As long as you have a STANDARD GSM-3G phone with the 2100MHz band used in Europe & Asia it will work fine whether roaming or if you can buy a SK Telecom or the other telecom carrier's GSM 3G SIM card.

In Japan this is NTT DoCoMo and Vodafone Japan (or the local name I forgot).

They ONLY have CDMA, 2 varients: CDMA2000 1X and EVDOrA and GSM-3G WCDMA/UMTS/HSPA+.

If you're from Europe your EU handset as long as it has 3G is good to go, just get it unlocked, it uses the same band.

If you're from the Americas make sure you have a GSM 3G phone with the 2100MHz band (same as Europe) and all the newer AT&T and T-Mobile phones HAVE THIS BAND.

If you have Verizon/Sprint there is also CDMA2000 1X/EVDO roaming there as long as you have a updated PRL (dial *228, option 2 on Verizon, Sprint is a different sequence) on the native network and you can roam there on the CDMA2000.

There is NO CDMA2000 CSIM card outside of those countries. If you're foreign you roam on the GSM 3G network (2100MHz UMTS/HSPA+, aka AT&T-TMobile technology).

AND why would you NOT want a GSM 3G phone, it allows access of 3 to 5x the amount of voice/data capacity.

Japan/South Korea are 1st world Asian countries, they were actually smart in getting rid of GSM 2G/EDGE very crappy spectral efficient technology.