For some reason I had an horrendous nightmare last night about a scenario where this didn't work and it got me thinking.
You may remember that they did this a couple of years ago maybe more so that all mobile phones would alert at once.
To my knowledge they've not tested it since. It's not that (apart from in my nightmares) I'm particularly expecting an emergency but wouldn't it be better for them to identify any issues that could have developed with the system before they actually need it? I'm not a techy but I could imagine things like phone software and security updates might have caused issues.