Mobile network O2 says it has fixed an issue affecting some customers across the UK which made it impossible to make or receive calls.
It is not clear how wide the technological issue was nor what caused it to happen.
Users reported problems on Friday morning.
An O2 statement on its status page read: "Our earlier issue with voice calls has now been fully resolved.
"We apologise for any inconvenience this may have caused."
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 an earlier statement, O2 said: "Our technical teams are investigating reports of issues when some customers are making and receiving voice calls.
How to reduce your Sky bill as TV, broadband and phone charges are set to rise in April
"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.
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.
EE customers can now get free Amazon Prime Video and stream it without using their data
GiffGaff, Tesco Mobile, Lycamobile and Sky Mobile compensation details after O2 crash
