Mobile phone provider EE has confirmed that some customers are currently experiencing problems making voice calls.
The firm said data and messaging services were working normally.
The fault appears to largely affect calls to non-EE phone numbers.
Nearly 3,000 people have left comments on the website Down Detector from around the UK, saying they have been unable to make or receive calls, on some occasions for several hours.
“Some of our customers are reporting problems when trying to make calls to some numbers this morning,” the firm said in a statement.
“All data and messaging services are working as normal. We’re working to fix this as quickly as possible and apologise for any inconvenience caused.”
EE also said on Twitter that emergency services numbers were still accessible.
Customers have taken to social networks and forums to complain.
“Can only call other mobiles in Nottingham, even local numbers aren’t working, sort this out EE, ironically I received a text from EE promoting BT Sports app during this downtime!” wrote Pat on Down Detector.
At the end of September the firm apologised again after a fault affected customers using its UK home broadband service.