Korean providers used to have only CDMA servicse but they now have both CDMA and WCDMA services.
Plus they started to add GSM support to their handsets so I was wondering if foreign SIM would work.
Both WCDMA based providers, SKT and KTF locked their phone so they would only work with SKT and KTF USIM. But some geniuse developed a tiny slice of circuits that works like unlocking. So I got one of them, YESSIM and put Softbank USIM into my LG-SH210 (Viewty.)
And voila it is working. SKT phones had little problems with voice calling. So I'd recommend to get KTF ones. They've been reported to work perfectly with both GSM and WCDMA networks.