Many on the Three network did not get anything, along with users on other networks – while some received multiple alerts. The government later said the message did not reach around 7% of compatible devices.
The Cabinet Office said at the time that the problems uncovered would be reviewed and addressed ahead of another test.
It said September’s test was intended to ensure the system worked well and to make sure people were familiar with the alerts, in line with other countries that also use them, like the US and Japan.
Of the approximately 87 million mobile phones in the UK, the alert will only appear on smartphones on 4G or 5G networks. Older phones, and phones connected to 2G or 3G networks, will not receive the message.
Cabinet Office minister Pat McFadden said: “Just like the fire alarm in your house, it’s important we test the system so that we know it will work if we need it.”
Addressing MPs in the House of Commons on Tuesday, McFadden said the alert system “will play a critical role in making sure that we are ready for all kinds of future emergencies”.
But some MPs highlighted concerns that text alerts and web information could fail to reach some citizens.
Liberal Democrat MP Sarah Olney said they could miss “millions of people – people without phones, without signal, without battery”.
Stafford MP Leigh Ingham meanwhile asked the government for assurances that the alert would reach rural constituents.
McFadden said roughly 95% of the UK population is covered by 4G or 5G access, and the government is working “constantly” with telecom providers to improve it.