I need a cell phone for 7 months

I just arrived for a 4-month student exchange, but will be staying in Korea for 7 months total. Tonight I went with some Korean friends to a cell phone shop and was told that because I am an exchange student (on a D2 visa), I can't get a cell phone even when I do get my alien registration card.

I am wondering if there is somewhere I can buy a used phone (I only need something simple) and then do pay-as-you-go?

Thanks!