At 3pm (UK time) on the 23rd April 2023, the government are sending out a public emergence released phone alert. This will be sent to pretty much all UK smartphones.
The phones will emit a sound, and contain a message saying – “This is a test of Emergency Alerts, a new UK government service that will warn you if there’s a life-threatening emergency nearby. “In a real emergency, follow the instructions in the alert to keep yourself and others safe.”
It will emit a siren sound and vibrate for 10 seconds even if the phone is on silent. You will need to acknowledge receiving this message before you are able to use your phone again for anything else. This will happen even if your phone is switched to silent!!

If for any reason you have a phone which you need to keep a secret for your (or other’s) safety, then you’ll need to go into your phone settings, and deactivate the alert system function beforehand, to ensure this doesn’t go off.
On this occasion they are purely testing the technology, and presumably the response to it. In future these types of alerts will be used for major events such as severe flooding, fires, and extreme weather.
You can find out more using this link. https://www.gov.uk/alerts
This link will take you to the BBC News Website for info too. https://www.bbc.co.uk/news/uk-65290166 and I have added a YouTube video below as well.