Get ready for a little chaos on Wednesday afternoon, because a test of the national Wireless Emergency Alerts will happen between 2:20 and 2:50 p.m. EDT. (That’s 11:20 to 11:50 a.m. on the West Coast.) If you have a burner phone that needs to stay hidden, or a baby who sleeps during that time, you may want to be prepared.

Why are they testing this alarm?

There’s a national Emergency Alert System, which provides those “this is only a test” messages you may have seen on TV or heard on the radio. It’s a descendant of CONELRAD, a Cold War era innovation meant to let the president tell us that World War III has broken out and Soviet bombers are dropping nukes as we speak. (CONELRAD rapidly switched transmitters to make it harder for those bombers to use radio broadcasts to navigate; once ICBMs were invented, this feature was unnecessary and the system was replaced with the more straightforward Emergency Broadcast System, and later the EAS.)

The EAS (for radio and TV) and the Wireless Emergency Alerts (for phones) are the same systems that are used for weather alerts, such as tornado warnings, and for Amber Alerts—so the system is very much in use. It’s just usually doing local or regional alerts, so from time to time, there’s a test of the entire national system. And that’s what will happen on Wednesday.

What happens on Wednesday afternoon?

At the same time—scheduled for 11:20 a.m. PDT, 12:20 p.m. MDT, 1:20 p.m. CDT, and 2:20 p.m. EDT—you can expect radio and TV stations to broadcast a test message, and wireless phones to sound an alarm.

If you have your phone on you, it will just be a momentary shock—the alerts tend to use a loud alarm sound rather than a notification ding or your normal ringtone. If your phone is not on you, it’s going to ring from its drawer or locker or wherever it lives. (I bet my kid’s middle school, where phones are allowed but must be kept in lockers or bags out of sight, is going to have a fun time with this.)

The cell tower will keep trying to send the message for 30 minutes, although each phone will only receive it once. So if you power on your phone at 2:25 (EST), you’ll get the alert then. After about 2:50 p.m., if all goes well, the test will be over.

Can I opt out of this alert?

Unfortunately, no. If your wireless provider participates in the WEA network—which they pretty much all do—they aren’t allowed to let you opt out of national alerts. (You can opt out of weather alerts and Amber alerts, though. Check your device settings if you’d like to do that.)

The alert will reach phones and phone-like devices (like some Apple Watches) if they are:

• Turned on

• Not in airplane mode

• Within reach of a cell tower (the message can’t reach you if you’re in a remote place with no signal)

• In an area where the wireless provider participates in WEA (so you won’t get this if you are overseas)

• A modern phone capable of receiving WEA alerts (some very old phones can’t)

If you have a phone that needs to stay quiet or hidden—including if this is your emergency line that you don’t want an abusive partner to know about—make sure that phone is turned off during the alarm time.