An investigation has been launched into why some mobile phone users in the UK failed to receive a message when a new national emergency alert system was tested on Sunday.
The alert was received by the "vast majority of compatible phones" during one of the largest public communications exercises carried out in the UK, the government said.
But some phones did not receive the 10-second alarm and message notification.
Users on the Three network were among those who failed to receive the alert during the test.
The network said it would work with the government to establish what happened.
Some social media used have said they have not been able to make or receive calls since the 3pm alarm went off on their device.
Engineers have not reported a trend of service interruptions, but officials are analysing the results of the test, the Cabinet Office said.
The emergency alert system is designed to warn the public if there of nearby threats, including flooding and wildfires.
"We are working with mobile network operators to review the outcome and any lessons learnt," a government said.
The alarm was scheduled to sound at 3pm on all devices using 4G and 5G networks in the UK.
For millions of phone users, the siren sounded for 10 seconds and a message was displayed to inform people that no action was needed during the test.
Some smartphones also read out the message aloud.
Some social media users reported receiving the message early, late or being sent more than one alert.
The UK's new Deputy Prime Minister Oliver Dowden said the test may be "annoying", but the system had the potential to save people's lives when it was put in place.
The test a "bit like when the fire alarm goes off at work", he told Sky News.
"It's a bit irritating at the time, but in the future people could be grateful for it because in a real emergency, this could be the sound that saves your life," Mr Dowden said.
He dismissed criticism of the system as an example of the government interfering in the lives of the public.
The Cabinet Office, meanwhile, said there was a spelling error in the Welsh language version of the alert message.
"A technical error caused one word in the Welsh language version of the emergency test alert to be misspelt," a government spokesman said.
"The fact that this occurred in a test alert will mean we can rectify it in future."
The entertainment and sport sectors had plans in place to limit disruption to large events during the test.
The London Marathon, Premier League football matches and matinee theatre performances were all taking place when the alert sounded.
