From The mirror:
"According to the
gov.uk website, the emergency alerts "work on all 4G and 5G phone networks in the UK" and you do not have to be connected to mobile data or wifi at the time to receive it.
However, you will not receive an alert if your device is:
- turned off or in airplane mode
- connected to a 2G or 3G network
- wifi only
Some devices simply may also not be compatible with the alert service - so how can you tell if yours is one of them?
It is said that the emergency alert will only work on
iPhones running iOS 14.5 or later and
Android phones and tablets running Android 11 or later. If your device is not running the latest software updates, be sure to install them on your
phone or tablet.
If you have an earlier version of Android, you may still be able to receive alerts.
To check, search your device settings for 'emergency alerts'."
NZ:
https://thespinoff.co.nz/business/16-02-2023/why-did-new-zealands-cellphone-network-fail-so-fast