The emergency alert test unleashed disruption today as runners at this afternoon's London Marathon were left staring at their phones in bewilderment and a BBC newsreader was interrupted live on air.
Footage from the finish line showed exhausted long distance runners who had just completed the arduous 26-mile route picking up their phones to get in touch with their loved ones when the loud noise abruptly sounded.
All people with 3G and 4G-capable mobile phones in the UK were supposed to receive a loud 10-second notification at 3pm on Sunday afternoon as part of a government test of a new emergency alert system - though customers on some networks reported receiving no notification at all.
Many people on social media described being taken off-guard as the shrill alarm struck interrupted their quiet Sunday afternoons, while pubs, supermarkets, theatres and places of worship became especially noisy as phones started to sound in unison.
Some London Marathon participants also noted the alert and loud sirens from spectator's phones ringing out around them as they took part in the annual race.
The BBC News channel was meanwhile interrupted live on air by the noise from the emergency alert, which could be heard ringing out in the studio.
Newsreader Richard Preston was discussing the alert live on air with a guest via video link when the noise started to be heard around him - though he did not comment on whether it was coming from his own phone.
Up at The Crucible in Sheffield, the Snooker World Championship was momentarily paused on Sunday to stop the alert from disrupting play.
Matches between Jack Lisowski and Anthony McGill and Mark Selby and Gary Wilson were stopped ahead of time in anticipation of the alert on devices in the auditorium, with the competition having strict rules against noise disruption from members of the audience.
Fans embarking on their way up Wembley Way ahead of this afternoon's FA Cup semi-final clash between Manchester United and Brighton were also momentarily distracted by the sound, with fans of both teams filmed staring at their phones in confusion.
Theatre productions taking place on Sunday afternoon were also interrupted by the alert. A play at The Lyric Theatre in London coincided with the interval, but a performance of Frozen at the Theatre Royal led to Olaf the Snowman reportedly joking he had set the fire alarm off.
The Cabinet Office said they would review Sunday's test to work out what went wrong after some people reported receiving the emergency alert late or not at all.
The department said that while the vast majority of compatible phones received the alert, officials were aware that a "very small proportion of mobile users on some networks did not receive it".
It said the issues would be looked into as part of the review of Sunday’s test.
A UK Government spokesman said: “We have effectively completed the test of the UK-wide Emergency Alerts system, the biggest public communications exercise of its kind ever done.
“We are working with mobile network operators to review the outcome and any lessons learned.”
Mobile network Three also said they are "aware" a number of customers did not receive the alert, and said they are now working with the government to try and work out what happened.
