A significant number of customers with the mobile network Three reported not receiving the emergency alert test earlier today.
The company said they are "aware" a number of customers did not receive the alert and they are working with the government to try and work out what happened.
In the future, the alerts could be vital for dangerous emergencies and newly appointed Deputy Prime Minister Oliver Dowden called the associated buzzing noise the "sound that could save your life".
The sound could be heard in the event of impending dangers like severe flooding, extreme weather, and fires.
A Three spokesperson said: “We are aware that a number of customers have not received the test alert. We are working closely with the government to understand why and ensure it doesn't happen when the system is in use.”
Cherished girl, 3, who spent half her life in hospital dies before surgeryThree said prior to the alert: "On Sunday 23 April at 3pm, the UK Government will be testing their emergency alert systems.
"Your phone will make a loud siren sound, even if it’s on silent, and you’ll see a message appear on your screen."
One person wrote: "[I] didn’t get an emergency alert. I was on Wi-Fi and mobile and have it turned on? Guess everyone on Three just dies in an emergency?"
The Cabinet Office said it would be reviewing the outcome of the UK-wide test of the new emergency alert system.
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 that 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.”