According to the Cabinet Office, while the “vast majority” of compatible phones received the alert, “a very small proportion” of mobile users on some networks did not receive it.
The alert was broadcast at 3p.m. on Sunday and should have been received by every 4G and 5G device across the UK.
Customers on the Three network appeared to be most affected by reports that some users did not see or hear the alert, which the firm acknowledged on Twitter.
“We’re working closely with the government to understand why and ensure it doesn’t happen when the system is in use,” it said.
Further still, a small number of people said that since the alert went off on their device, their phone had not been able to make or receive calls.
In Wales, the test was broadcast in the Welsh language but contained an error. For the words “others safe”, the message read “eraill yn Vogel” when it should have been “eraill yn ddiogel”.
The UK government blamed a “technical error” for the...