The system can alert the public to threats ranging from severe flood warnings to fires, explosions, terrorist incidents or public health emergencies.
On Tuesday, 22 June, between 1pm and 2pm BST, the UK government – in collaboration with the country's mobile carriers – will be conducting an initial test of the system that's slated to affect Android phones. A secondary, localised test for iPhones is scheduled to take place on 29 June, across the Reading, Berkshire area - as confirmed by MacRumours.
The alert in question is expected to include a siren-like sound and vibration for around ten seconds, with some notices potentially also including a phone number or a link to the GOV.UK website, where more information can be found. The alerts (including the siren) are said to bypass a phone's silent or vibrate settings.
Those who don't wish to receive such notifications going forward will be able to opt-out, depending on the severity of the warning being issued, however, the most important warnings will be pushed out to devices regardless of user preference.
Android users can toggle which types of emergency alerts they'll receive by searching for the 'emergency alerts' or 'wireless emergency alerts' menu in their phone's settings, while iOS users will an emergency alerts section under 'settings' > 'notifications', with toggles for both 'Extreme Alerts' and 'Severe Alerts'.