An emergency alert arrives on every handset in an area at once, overriding silent mode. It does this by using a broadcast channel that behaves nothing like a text message.
Broadcast is not messaging
An ordinary message is addressed to one subscriber and delivered individually. Sending millions of them would take time the network does not have during an emergency.
A cell broadcast instead transmits a single message from each cell tower to every handset listening to it, in the same way a radio station reaches every receiver in range.
Because the network sends one copy rather than millions, delivery is nearly simultaneous and does not degrade as the number of recipients grows.
Congestion does not block it
During a serious incident, voice and data capacity is often saturated by people calling one another. Individually addressed messages queue behind that traffic.
Broadcast alerts use a separate control channel that is not competing with ordinary traffic, so they arrive when calls are failing and pages will not load.
This separation is deliberate. The channel is reserved precisely because the moment an alert is most needed is the moment the network is least able to carry normal traffic.
Targeting works by geography, not by subscriber
Authorities select an area, and the alert is transmitted by the towers covering it. Every compatible handset within reach receives it regardless of who owns the device.
The system therefore has no list of recipients and needs none. Visitors and roaming phones receive the alert alongside residents, without any registration step.
Precision is limited by cell coverage. Towers do not respect administrative boundaries, so alerts commonly spill into neighbouring areas, and the wording usually accounts for that.
Severity levels govern the behaviour
Alerts are graded, and the grade determines whether the handset sounds a distinctive tone, overrides silent settings, or simply appears as a notification.
The highest grade is reserved for immediate threats to life, because an override that fires too often trains people to dismiss it without reading.
Lower grades can usually be switched off in handset settings, while the top grade generally cannot, which is the source of most public confusion about the system.
Authorisation is deliberately narrow
The ability to originate an alert is restricted to a small number of designated agencies, with technical controls limiting what each may send and where.
That narrowness is a safety feature. A channel that overrides every phone in a region is only tolerable if the list of people who can use it is short and accountable.
Test transmissions exist for the same reason. Regular tests confirm the chain works end to end, and they familiarise the public with a sound they should not have to interpret under pressure.