INARAJAN, Guam — Against the back wall of the command center at Guam Homeland Security, a nondescript telephone is perched on a shelf. It’s the phone no one in the room wants to hear ringing: It alerts Guam to an incoming ballistic missile.

A call on this phone would only come from the United States Pacific Command in Hawaii to inform Guam of the impending strike.

If it were to ring, a blue light would flash and immediately set into motion a chain of emergency response procedures to alert all of Guam’s roughly 162,000 civilians of the threat within two minutes. The system includes mass notification sirens that are positioned around the island, radio and television emergency broadcasts, and emergency medical workers and mayors equipped with mobile public address systems.

Workers at the Homeland Security office have been on 24-hour duty fielding questions from residents and the news media since North Korea warned last week it was preparing a missile test that would create an “enveloping fire” in the waters off Guam.