Some O2 voice calls are not working properly, leaving people unable to talk on the phone.
The network has confirmed it is aware of the issue and is working to fix it.
"We’re aware of an issue impacting some voice calls," an update on the network's status page reads.
"Our technical teams are investigating. We apologise for any inconvenience."
It did not suggest that there were any issues with other parts of the network, such as text messages or data connections.
The tracking website Down Detector showed many thousands of people reporting the issues. Problems came from across the UK, the site showed.
Reports began around noon local UK time, and have continued since.
Mobile networks and internet providers have largely kept connections online through coronavirus lockdowns, but some have been hit by issues.
Last month, as the lockdown measures went into effect, O2 and other networks stopped working properly as the increased usage led to strain on its infrastructure.
