A national alert test will buzz US TVs, radios, and any phones that opt-in at 2:20PM ET
At approximately 2:20PM ET, FEMA will conduct a test of both the Emergency Alert System (EAS) and the Wireless Emergency Alerts (WEA) covering radio, TV broadcasts, and mobile devices in the US, but you might not see it on your phone. That’s because this time around, unlike the Presidential Alert test in 2018, it will only reach phones with a setting to receive test messages and by default, most phones are set to opt-out.
According to this document (PDF) from the FCC, on Android phones, there should be a Wireless Emergency Alerts section in your settings menu with an option for “State/Local Test alerts” that you can turn on or off. The process described for iOS is more complicated, but Apple’s support page explains how you can opt-in: