There was a nationwide alert on all smartphones in the UK today at 3pm. I didn’t get it, that’s me not surviving the next extinction level event then 
TBH I had forgotten about it, it was only when it went off at the Snooker (I was watching on TV!) that I checked my phone hadn’t run out of battery (it hadn’t).
Don’t worry, there were so many doomsdays in last 20-30 years it is surprising anyone is still alive today. 
It appears to be a problem with my provider Three.
Keep poking the bear long enough anything can happen.
Saw a lot of cracks on YouTube comments about how that alert was really for how thoroughly Spurs were getting stomped into snot by Newcastle.
There also should be a setting somewhere in your phone to enable/disable it. Maybe it’s disabled.
Nope, seems to be a problem with Three.