On Sunday 23rd April at 3pm there will be a national test of the UK Emergency Alert service. This a UK government service that will warn you if there is a danger to life nearby, such as severe flooding, extreme weather or fires.
It will involve phones emitting a ten second sound alert and displaying an emergency alert message on your home screen with advice about how to stay safe: https://www.youtube.com/watch?v=rmdqHLekkBQ&t=1s
Emergency Alerts require no personal information as the technology used allows a message to be broadcast to a defined area, meaning any compatible device in or entering that area immediately receives the message.
It is possible to opt out of receiving Emergency Alerts in your phone’s settings, however it is hoped that people do not as the service is intended to be used only to warn you when lives are in danger.
More information is available at: https://www.gov.uk/alerts/how-alerts-work