Chingu mobile is one of the best and chepeasr telephone service providers for foreigners in South Korea. They were very helpful to me, I sent them the order request through KakaoTalk as I couldn't do my payment online(Because I didn't log in to Paypal), you can do through online or through Facebook as well. Helped me through each step until my sim was activated. They sent a message as soon as I landed as well to check if I have reached Seoul. 

Great and friendly service, affordable prices.

My plan- 2 months 30000Won