South Korean Intelligence and it's allies presumably have agents in the NK military who would give them advance notice.
NK can just set up their own cell phone receiving SK alerts and know that they know and cancel the strike, making it into a boy that called wolf situation and possibly exposing agents.
alt Hacker News
NK can just set up their own cell phone receiving SK alerts and know that they know and cancel the strike, making it into a boy that called wolf situation and possibly exposing agents.