There's a lot of GSM literature about SMS (and MMS) protocols. Including SMS-CB or CB-SMS (Cellular Broadcast modes, sent from the network to all connected compatible devices). Cellular Broadcasts can be at national, provincial, or regional (municipal) levels - and they're unselective, if your device has power and is compatible then it gets alerts (regardless of your carrier or your country), if your device is unpowered or out of range then it gets no alerts.
It's (slowly) becoming internationally standardized. Canada's flavour is Alert Ready. Now officially operational in Alberta, British Columbia, Saskatchewan, Manitoba, and Quebec - unofficially operational in Ontario - plus some other coverage zones, minus some blind spots. Initially proposed by the Big Three telcoms and the CBC (and strongly opposed by the CRTC), ironically now a CRTC-regulated gongshow (which the CBC uses very reluctantly and Big Four have made the CWTA advocate repeatedly for allowing customer opt-outs).
Some customers (and the CRTC) argue that the system can save lives. Some customers don't want random alerts about faraway emergencies (or tests) forcing their phones to override all user controls with distinctly alarming and annoying noises.
Some SMS apps used to provide user settings to block SMS-MT/SMS-OT (paid network advertisements) and SMS-CB (emergency broadcasts, disaster response, national defense, extreme weather warnings, amber alerts, etc) - sometimes as generic on/off, sometimes as specific categories. The CRTC has disallowed all SMS-CB blocking features in Canada (because it could save lives!) so if you want to block alert broadcasts (you insensitive monster!) then you'll have to get a little creative in finding the "old" versions of these apps (or compile your own SMS-CB blocking app from modded github sourcecode).
https://www.alertready.ca/
https://en.wikipedia.org/wiki/Alert_Ready
