http://www.knra.or.kr/M4/data/05_04-VoIPmarket.pdf
http://mushman.co.kr/entry/%ED%8E%98%EC%9D%B4%EC%8A%A4%EB%B6%81%EC%9A%A9-VoIP-%EC%84%9C%EB%B9%84%EC%8A%A4-%EB%B4%87%EB%AC%BC