An alert sounds on one phone and not on the identical model beside it. The system does not send messages to people, and understanding what it does send explains almost every gap in coverage.
Alerts are broadcast, not addressed
A wireless emergency alert is not a text message. No phone number is involved, and no list of recipients exists anywhere in the system.
An authorised agency sends the message to a federal gateway, which passes it to participating carriers, who transmit it from specified cell sites.
Every compatible handset connected to those sites displays it. The delivery target is a set of towers rather than a set of subscribers.
Tower geography rarely matches the danger zone
The area an agency draws on a map must be translated into a list of cell sites, and cell coverage areas are irregular shapes determined by terrain and antenna placement.
Because a partial match is safer than a miss, the selection typically errs toward including towers that reach beyond the intended boundary.
That produces both outcomes people complain about, which are alerts received well outside the affected area and, occasionally, gaps just inside it.
Which tower a phone uses is not simply the nearest
A handset connects to the site offering the best signal, which may be several miles away across a valley while a closer one is blocked by a hill or a building.
Two phones in the same room can therefore be attached to different towers, particularly indoors and near a coverage boundary.
If only one of those towers was included in the broadcast, only one phone alerts.
Device and network settings change the result
Handsets allow certain alert categories to be disabled, though the highest-level national alerts cannot be switched off.
Older devices, phones connected only to a foreign network while roaming, and handsets with no active service may not receive the broadcast at all.
Airplane mode and a phone connected solely over wireless internet also sit outside the mechanism, since the alert travels over the cellular broadcast channel.
Message length and language are constrained by the channel
The broadcast channel was designed for short bursts, which is why alerts are terse and why longer formats and links were added only as networks were upgraded.
The same constraint limits how many languages an alert can carry, and translation availability varies by carrier and device rather than by the sending agency. These limits are why local officials still pair the system with sirens, calls and door knocking.