By Karen Zraick

Oct. 3, 2018

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

Officials said before the wireless test they believed that it would reach about 75 percent of the cellphones in the country, though they hoped the number would be higher. It could take up to 30 minutes for the alerts to be transmitted to all devices.

Some things that could interfere: ongoing phone calls or data transmission, a device that is turned off or out of range, and smaller cellphone providers that are not participating in the program.

The test, originally planned for last month but delayed by Hurricane Florence, was the culmination of many years of work. The federal government developed a system to issue the alerts, which are scripted in coordination with numerous government agencies. They are limited to 90 characters, but will be expanded to 360 in the future.

https://www.nytimes.com/2018/10/03/us/presidential-alert-trump.html