Mobile network O2 is investigating an issue affecting some customers across the country which is making it impossible to make or receive calls.
“We’re aware of an issue,” the company’s coverage status page reads.
“Our technical teams are investigating. We apologise for any inconvenience.”
It is not clear how wide the issue is nor what is causing it to happen.
A number of users began contacting the network via Twitter on Friday morning, citing that they are unable to make calls to others, as well as problems receiving them.
In a statement, O2 said: “Our technical teams are investigating reports of issues when some customers are making and receiving voice calls.
“Data use is not impacted. We apologise for any inconvenience.”
Other mobile networks, including Sky, Tesco and Giffgaff rely on O2’s services, but there have been no reported issues from their customers as yet.
The incident comes after a major outage in December, in which millions of customers were unable to get online for at least a day.
O2, which has more than 25 million UK customers, and its systems and equipment provider Ericsson issued a joint apology for last year’s outage, saying it was a result of a software issue.