Millions of phones across the UK sounded with a siren on Sunday as part of a test for a new national emergency alert system.
The loud alarm was planned to ring at 3pm on all devices that were using 4G and 5G networks in the UK.
The alert rang for 10 seconds and displayed a message notifying phone users that no action was needed in response to the test.
Some smartphones also read out the message to recipients.
However, others said their phone did not display the message or make a sound.
Phones that were powered off or switched to airplane mode were not expected to sound.
The emergency alert system is designed to warn the public if there is a danger to life nearby.
In future, a similarly loud notification and message will be sent to those the UK Government is seeking to reach.
Once established, the system is intended to be used in life-threatening situations including flooding and wildfires.
Speaking before the test, Deputy Prime Minister Oliver Dowden conceded the drill could be “annoying” but that it had the potential to save people’s lives once rolled out.
He told Sky News that the trial run was a “bit like when the fire alarm goes off at work”.
The Cabinet minister denied the new system was an example of nanny state interference, telling the BBC he did not accept “that characterisation”.
People who do not wish to receive future alerts will be able to opt out using their device settings but officials hope the life-saving potential of the messages means users will keep them on.
The test message that appeared on phones said: “This is a test of Emergency Alerts, a new UK Government service that will warn you if there’s a life-threatening emergency nearby.
“In a real emergency, follow the instructions in the alert to keep yourself and others safe.
“Visit gov.uk/alerts for more information.
“This is a test. You do not need to take any action.”
The entertainment and sport sectors had been planning how to guard against disruption to large events when the test went off.