교환원의 도움 없이 사용자가 직접 해외 전화번호를 눌러 연결하는 가입자 직통 국제 전화 시스템.
A system that allows a telephone subscriber to make international calls directly without the assistance of an operator.
예문:
"The introduction of ISD services revolutionized global business communication in the late 20th century."
"Check the ISD rates before calling your relatives abroad to avoid unexpected charges."