Might be worth adding a reminder to your calendar (to reduce being startled):
"The national system for sending emergency alerts to mobile phones in the UK will be tested again this September, the government has said.
It will see compatible phones vibrate and make a siren sound for 10 seconds while displaying a message at 15:00 BST on 7 September, even if they are set to silent."
It depends which service it's using - only phones connected to 4g or 5g will get it. 3g has been disconnected, but there are some phones still connected to 2g which won't receive the alert.
