The UK government's emergency alert system test will sound on Sunday 23 April at 3pm,
The sound may be different, depending on the device, but the government says it may make a loud siren sound (even if the phone or tablet is on silent).
It could also vibrate, or even read out a message automatically.
The alert will last around ten seconds and you'll need to swipe the message to clear it or hit "OK" and carry on using their device.
People who have their phones switched off will not receive the message.