(NEXSTAR) – A nationwide test of the country’s Emergency Alert System Wednesday went out to TVs, radios and phones – at least most phones.

The test was a joint effort by the Federal Emergency Management Agency (FEMA) and Federal Communications Commission (FCC), making sure they have ways to alert the country in case of national emergencies. Those who received the test heard their phone blare or felt it vibrate for a few moments starting at around 2:18 p.m. Eastern Time. (The test was scheduled to start at 2:20 p.m., and end half an hour later, but the alerts started a couple minutes early.)

All major U.S. wireless providers that participate in the emergency alert system transmitted the national test to their customers.

If your phone didn’t go off, it may simply be because it was turned off, on airplane mode, or out of range of an active cell tower.

You may have also been on a call at the time. “The national test will not interrupt a phone call,” FEMA says.

Older phones may also not be compatible with Wireless Emergency Alerts, or WEA. Verizon has a list of phone models that are new enough to receive and display the alerts. For iPhones, for example, you’ll need an iPhone 4 or later to get any sort of alert. iPhones XR and later work with the latest version of WEA, which has improved geographic targeting in case of a real emergency.

If your cell coverage is from a smaller provider, they may also have opted out. “Some providers do not participate in WEA and will not transmit the national test,” FEMA says.

Federal law requires the FCC and FEMA test the system at least once every three years. The last time it was tested was in August 2021.