On Wednesday (03.10.18), a message will be sent to everyone using a phone in the US running on a network operated by a carrier participtating in the system.
The test message from the Integrated Public Alert and Warning System reads: "Presidential Alert: THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed"
The Federal Emergency Management Agency system - the team behind the Amber Alerts sent to phones - is also sending out the WEA test.
The message will be sent out at 11:18 a.m. PT - or 2:18 p.m. ET - although it could take a few minutes for it to arrive on all phones.
Meanwhile, a description on the FEMA website notes: "WEA includes a special tone (some describe it as quite loud) and a vibration, both repeated twice."