O2 customers in parts of the country have been left unable to make calls because of a technical issue.
On Sunday, a number of people using O2 said they could not make outgoing calls despite appearing to have full bars of signal.
Customers on social media said restarting their device and switching off the connection did not help with the issue.
Downdetector, which collects data from phone users across the UK, suggested the most affected areas were London, Bristol, Brighton and Preston.
It is thought that O2 customers are still able to text and use 4G during the issue with calls.
A spokesman for O2 said on their website: "We are aware of an issue in some parts of the country.
"Our technical teams are investigating. We apologise for any inconvenience."
The spokesman did not suggest when the issue may be fixed.
