The nationwide emergency alert test conducted at 15:00 BST on Sunday failed on some phones, resulting with many people not receiving the alarm.
The government has said it will launch a review after many smartphone users complained they did not receive the nationwide emergency alert test.
Many people on social media reported the alert went off one minute earlier or later than planned, when a loud, buzzing alarm rang out and the words “severe alert” flashed up on their phones’ screens.
And some said they did not receive the alert at all.
A Cabinet Office spokesperson acknowledged that the alert did not reach some mobile phones, adding it “will be looking at this as part of our review of the test”.
The department also said engineers had spotted a trend of phone functions failing to work afterwards, adding that officials were in the early stages of analysing the results of the trial run.
Many users of the Three mobile network said they did not receive the alert.
A spokesperson for the company later said: “We’re aware that a number of customers have not received the test alert.
“We’re working closely with the government to understand why and ensure it doesn’t happen when the system is in use”.
Andrew Hamilton, a member of the UK Youth Parliament for North Down, who is also blind, called the alert “inaccessible”.
He told BBC News: “I use a screen reader but because of the alert sound, the screen reader couldn’t tell me what the alert was about.”
The government had previously advised that those with 2G or 3G devices would not get the alert, along with phones that were off or in aeroplane mode.
People were further advised to ensure their phones had all the latest software updates, including iPhones running iOS 14.5 or later and Android phones and tablets running Android 11 or later.
In Wales, the alert included a translation error as the words “others safe” appeared in the message as “eraill yn Vogel” rather than “eraill yn ddiogel” in Welsh.
“Vogel” does not mean anything in Welsh, as there is no letter V in its alphabet – although Vogel is a ski resort in Slovenia.
Meanwhile, the National Centre for Domestic Violence warned people with hidden mobile phones to turn off the alerts to avoid revealing the location of any secret devices.
People who do not wish to receive future alerts can opt out using their device settings.