Taken from online sources:
This Sunday (23 April 2023) at 3pm the Government will be testing their new Emergency Alert system. This is a test of Emergency Alerts, a new UK Government service that will warn you if there's a life-threatening emergency nearby. This loud alarm will sound even if your phone is switched to silent or notifications are off.
If you're living with domestic abuse and have a mobile phone hidden in your house you may want to switch off to avoid discovery.