On Wednesday, phones across the country lit up with a special message from the president as FEMA tested its wireless alert system.
Getting a message out to this many phones has never been done before. For a brief moment, a huge chunk of U.S. cellphone infrastructure was all doing the same thing.
With this test, FEMA will make sure all those different parts work the way they're expected to so it can fix any parts that don't, in case they're ever needed for real.
Because things are usually bad enough for the cellular network during an emergency. Natural disasters frequently knock out power to cell towers, sometimes over a wide area. And even when the towers stay on, the increased traffic while everyone tries to get messages out can overload the system.
And in one recent case in California, the common practice of throttling data plans hobbled the network that wildland firefighters needed in the field.
Verizon says it won't clamp down on emergency responder networks in the future. AT&T is deploying cell sites that can fit on a drone to bring up temporary networks in disaster zones.
The FCC encourages text messages like the presidential alert in the event of an emergency. Text doesn't need as much data as a phone call, which makes alerts more likely to get through — and that could give people more time to respond.