This from the BBC news site:
A siren-like alert will be sent to smartphone users across the UK next month to test a new government public warning system.
It allows the government and emergency services to send urgent messages warning the public of life-threatening situations like flooding or wildfires.
The test is expected to take place in the early evening of 23 April.
Phone users will have to acknowledge the alert before they can use other features on their devices. 
A message will appear on the home screens of people's devices during the test, with vibration and a loud warning sound that will ring for about 10 seconds, even if the phone is set to silent.
The system - which became operational on Sunday - is being modelled on similar schemes used in the US, Canada, Japan and The Netherlands. Emergency alerts work on all 4G and 5G phone networks and will not include older "non-smart" phones, the government said.
Messages would only ever come from the government or emergency services and will initially focus on the most serious weather-related events, with the ability to get a message to 90% of mobile users within the relevant area.
Terror alerts could be added to the list of potential events that would trigger a notification.
The messages will include details of the area impacted and instructions about how to respond. They will only be sent where there is an immediate risk to life and many people may not receive an alert for months or years.
Cabinet minister Oliver Dowden told the BBC the warnings are sent in a "very targeted way" and other than the test, he hopes many people will never hear the alert again.
People can opt out by searching their device settings for emergency alerts and then turning off severe and extreme ones. Officials say the alerts could be life-saving, though, advising against switching them off.