(CNN) If you have a cell phone, it probably buzzed and beeped loudly Wednesday afternoon around 2:18 p.m. ET.

"Presidential Alert," the screen said. "THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed."

It's the first nationwide test of the system built by the federal government and cell phone carriers to warn Americans of an emergency, like a terror attack or a widespread disaster.

We've seen Amber Alerts for missing children and weather alerts for incoming thunderstorms or tornadoes. The presidential alert is like those, only it goes to virtually every cell phone in the country, and you can't turn it off.

What is this?

The federal government has long had a system to issue alerts over television and radio. It has grown more sophisticated since it was created in the 1950s, and now includes a system to send warnings to cell phones, too. (The TV and radio system is tested monthly.)

The buzz and tone have the same feel and sound as those used for the Amber and weather alerts.

The system is run by the Federal Emergency Management Agency, which says all of the country's major cell phone carriers participate.

The test was originally scheduled for September but was postponed as FEMA responded to Hurricane Florence. The law behind the cell phone notification system, which was passed in 2016, requires FEMA to run a test at least every three years.

Is it a message from President Trump?

It's called the "presidential alert," but the President doesn't actually write it.

Instead, FEMA officials confer with other government agencies and the White House, select one of several pre-written messages, customize the message to fit the particular emergency and send it out.

"The President will not originate this alert, say, from his mobile device," a senior FEMA official told reporters on Tuesday. "You would not have a situation where any sitting president would wake up one morning and attempt to send a particular message."

Then who pushes the button?

If you're picturing a red button inside a glass box or a fire-alarm-style handle on the wall -- this isn't that.

FEMA officials use a device that's "very similar to a laptop computer," the senior FEMA official said. After filling in the message form, two other FEMA officials are asked to sign off on the alert -- a system designed to prevent false alarms, like the incorrect alert of an incoming missile that roused and terrified people in Hawaii earlier this year.

"Everything is secured, password-protected and then authenticated or checked by two people before that message is sent," the official said.

Can I silence my phone or turn off the alert?

Unlike the Amber and weather alerts, the presidential alert can't be turned off.

Because the message is meant to include "critical lifesaving information," it's designed to reach as many cell phones as possible, even those that aren't activated or don't have SIM cards.

The FEMA official said that in past, smaller tests, about 75% of phones received the alert. If a phone is off, or on a call at the time of the alert, the message many not come through.

What about the other screens in my life?

If you were looking at another piece of glowing glass -- like your television -- during the test, you may have seen the alert there, too.

Here's what you may have seen on TV or heard on the radio: "THIS IS A TEST of the National Emergency Alert System. This system was developed by broadcast and cable operators in voluntary cooperation with the Federal Emergency Management Agency, the Federal Communications Commission, and local authorities to keep you informed in the event of an emergency. If this had been an actual emergency an official message would have followed the tone alert you heard at the start of this message. A similar wireless emergency alert test message has been sent to all cell phones nationwide. Some cell phones will receive the message; others will not. No action is required."

The message was also scheduled to appear on some smartwatches connected to cell phones, FEMA said.

What about my privacy?

The FCC said it does not collect data based on the test, though it will ask cell service providers for feedback about how the test went.