An emergency alert test will hit all phones Wednesday. How the wireless system works.

A nationwide emergency alert test scheduled for Wednesday may feel as if it's coming from everywhere. The test, led by the Federal Emergency Management Agency and the Federal Communications Commission, is scheduled for 2:20 p.m. EDT and will involve all agencies who are part of the Emergency Alert System, wireless providers and emergency managers, the agencies said in a joint release.

The test will be comprised of two portions simultaneously – the Emergency Alert System for televisions and radios, and Wireless Emergency Alerts for enabled devices that are turned on and in range of an active cell tower.

The test message will look similar to the regular Emergency Alert System test messages:

“This is a nationwide test of the Emergency Alert System, issued by the Federal Emergency Management Agency, covering the United States from 14:20 to 14:50 hours ET. This is only a test. No action is required by the public."

How do Wireless Emergency Alerts work?

Wireless Emergency Alerts are like traditional emergency alerts but are sent out over a network of wireless carriers' signals. The National Weather Service often uses this system to alert the public ahead of imminent severe weather.

Here's a quick look at how wireless alerts typically work:

In a regional alert, an agency like the National Weather Service identifies a zone in which alerts are necessary to warn people of imminent danger, like severe weather. Cell towers within range of that area send out an alert.

In a national emergency – and like the Wednesday joint test by FEMA and FCC – the wireless emergency alert can alert all cellphones that are turned on and in range of an active cell tower. Phones that are set to Wi-Fi or airplane mode will not receive the alert.

What does the alert sound like?

Because of an FCC rule prohibiting the alert sound's "use of in any circumstance other than in an actual National, State or Local Area emergency or authorized test," we can't play the actual warning sound. So here's FEMA's description of how it sounds:

From Title 47 of Code of Federal Regulations

FEMA says Wednesday's test will last for approximately one minute and will go out only once, with no repeats.