The UK government will test the Emergency Alerts system on 7 Sep 2025 at 3pm, causing a loud siren sound on your phone, even if silent.
For more information – About Emergency Alerts – GOV.UK
If you are a domestic abuse victim with a concealed phone, you can opt out –https://www.gov.uk/alerts/opting-out
Please also see leaflet – UK Emergency Alert Test 2025