I hav a mobile phone that I used in Europe.
It's an unlocked GSM triband phone.

Is this what is used in Korea? Can I just go to a local dealer and get a SIM chip for it? Or are you on some other system there?

Unfortunately, no. The RoK uses a different type of cell coverage. From what I've read within this forum, the best bet is to buy a used one from one of the many dealers in country then buy pre-paid calling cards for local calls. (I'll find out in 2 months when I get there....)

I brought a phone with me here that was supposed to be able to work on the system they use here but the cards were no where near the same size.

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There's No Toilet Paper on the Road Less Traveled.

My triband phones purchased in Germany are sitting in a drawer waiting for us to go back to Germany. They don't work in Korea, however, we had no problem calling from the Great Wall of China. Best thing to do is shop around for a phone here and buy the pre paid cards. BTW you can get some really cheap deals on the phones.