An emergency alert arrives on every phone in an area simultaneously, without anyone having registered. It works because it is not a text message at all.
Text messaging could not do this
A text message is addressed to one number and delivered individually, so reaching millions of handsets means sending millions of separate messages.
Networks process those messages in sequence, and a mass send would take a long time and congest the network precisely when it is most needed.
It would also require a list of every number in the affected area, which does not exist and could not lawfully be assembled for the purpose.
Cell broadcast sends once to everyone
Alerts instead use a broadcast channel, in which a mast transmits a single message that every compatible handset in its coverage listens for.
The network sends one transmission regardless of how many phones receive it, so delivery time does not increase with the number of recipients.
No subscriber information is involved, and the network does not know which handsets received the alert, since nothing is sent back.
Geography is defined by which masts transmit
Targeting works by selecting the masts that serve the affected area and broadcasting only from those.
Precision is therefore limited by cell coverage, and alerts routinely reach people just outside the intended zone where a mast's range extends beyond it.
Handsets in the area receive the alert whether or not they are registered locally, which is why visitors and travellers are covered automatically.
Severity levels behave differently
Systems define several categories, and the most serious override silent settings and sound a distinctive tone that cannot be mistaken for a notification.
Lower categories can usually be disabled in a handset's settings, while the highest level is generally not optional.
Which categories exist, who may authorise a message and what may be disabled are all set nationally and differ considerably between countries.
Overuse is the main risk
The system depends on people paying attention, and attention degrades quickly if alerts arrive for events that do not warrant them.
Authorities therefore restrict use to defined threat levels and test the system openly, so a scheduled test does not train people to ignore the tone.
Alerts that reach far beyond the affected area cause the same problem, which is why targeting accuracy matters as much as the technology itself.