At 3pm yesterday, millions of phones across the UK blared out a 10-second siren in unison.
This was part of a much-hyped Government test of the new emergency alert system, designed to spread urgent warnings in a more localised and speedy way than SMS.
However, many Londoners didn’t receive the alert — and not only those who have disabled the feature. Here are some of the reasons your phone may not have sounded the alarm along with everyone else.
You’re on the Three mobile network
Many people who didn’t receive the alert test have something in common: they are on the Three mobile network.
Three has acknowledged there was a problem with its delivery. “We’re aware that a number of customers have not received the test alert,” the company wrote on its official Twitter account. “We’re working closely with the Government to understand why and ensure it doesn’t happen when the system is in use.”
No official follow-up explanation has been given, but one theory posited by developer David Wheatley on Twitter was that Three’s implementation was simply different from other networks.
“Networks who successfully performed the trial did rebroadcast the signalling (SIB12-v920) continuously for the full 20-minute duration of the test, stopping the alert at about 15:21,” he wrote. “This meant, if you had a device in airplane mode until 15:15, you’d still get the alert.”
This is probably what happened again this time, which would explain the variety of people to received the alert and who didn't.
— David Wheatley (@davwheat_) April 23, 2023
In a nutshell, every Three site likely sent the alert only once, and devices don't constantly look for these messages, hence why they should repeat.
By contrast, Wheatley’s analysis suggests Three sent the alert only once. This meant phones not actively looking for the signal during that brief window will have missed it. “Devices don’t constantly look for these messages, hence why they should repeat,” he said.
You weren’t connected to 4G or 5G at the time
The emergency alert system relies on 4G and 5G connectivity. If your phone was in an area without reception — either in the sticks with 3G connectivity, or below ground without a good signal — then the alarm simply won’t have sounded.
This also applies if your phone was on airplane mode at the time of the alarm, too.
Your phone’s operating system is outdated
While both Android handsets and iPhones can receive emergency alerts, they have to be reasonably up-to-date to do so. iPhones must be running iOS 14.5 or later. While the iPhone 13 and 14 were shipped with the feature ready to go, earlier handsets would need to have been updated at some point in the last two years. Handsets before 2015’s iPhone 6S are incompatible.
Android devices, meanwhile, need to be running Android 11 or later. That version arrived in 2020, though older versions “may still be able to receive alerts”, the Government says.
You’ve disabled emergency alerts on your phone
Some people disabled emergency alerts on their phone before the test. This was over fears that the feature would be abused or that it’s a sign of Government overreach.
This is very hard to do by accident, however, requiring you to dig into your phone’s notifications settings menu to opt out.
