At 2:18 p.m. Eastern on Wednesday, cellphones across the United States emitted the ominous ring of an emergency presidential alert.

It was the first nationwide test of a wireless emergency alert system, designed to warn people of a dire threat, like a terror attack, pandemic or natural disaster.

“THIS IS A TEST of the National Wireless Emergency Alert System,” it read. “No action is needed.”

Two minutes later, televisions and radios broadcast test alerts. There was no notification plan for landlines.

The president — or someone he designates — would make the decision to send a real alert in case of a nationwide catastrophe, and the Federal Emergency Management Agency would push the button.