O2 customers have reported ongoing issues as the network says one of its phone masts has "stopped working".
Outage monitoring site Downdetector reported a spike in customer complaints at around 2.30pm on Sunday, as Twitter users vented their frustrations.
Several said they had been unable to make or receive phone calls.
A map on the site suggested the issue was affecting customers across the country, with London, Leeds and Liverpool among the cities logging the most complaints.
One man in the south-east of the capital tweeted: "@O2 what's happening with the network in SE London? I've not been able to make calls for the last hour!"
While a woman posted a screenshot of a message on her phone reading: "O2 has turned off 4G calls on your account".
A fellow confused customer tweeted to the network provider: "Hi. I’ve just had a pop up saying O2 has removed 4G calling. Is this a glitch?"
Shortly after 3pm, an O2 representative explained that they had looked into the issue and discovered one of their phone masts had stopped working.
A spokesperson for the network later issued a statement saying: "We’re aware that a small proportion of customers who use voice calls over 4G data or Wifi may have experienced issues in the last hour.
"Customers who do not use these services are not affected and O2 data is not impacted. Customers can continue to use 3G and 2G to make calls as normal."
At around 4pm, an O2 representative told the Standard that the issue "has now been fixed."
It comes less than three weeks after customers reported being unable to make outgoing calls owing to a technical issue.
