https://techcrunch.com/2018/10/03/fema-is-about-to-send-a-presidential-alert-to-millions-of-u-s-phones/

In a few hours, millions of Americans will get a test emergency “Presidential Alert” message sent to their phone — a simulation in case the president ever needs to reach to entire country in a national emergency.

At 2:18pm ET, the Federal Emergency Management Agency will send a short alert, saying: “THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed.” A few minutes later, televisions and radio broadcasts will briefly suspend and a similar message will run.