It didn't work on mine, Three.
I'm not sure if being connected to the wifi stopped it?
Worked on my S10 on Three, didn't work on my mums Pixel 4 on O2. Wouldn't be shocked for her to get it in 3 days time, they've done that with SMS before (though obviously this isn't SMS, it should be priority).
I also was surprised it wasn't louder.
Ok .
I give the system 6 months before the hackers get into it.
As I recall there used to be a way to send network messages to phones that pop-up like this, sort of like a special priority SMS.