O2 resolves fault which left customers having problems using phones
In an updated statement following the issue which affected customers across Britain on Friday morning, the company said: "Our earlier issue has now been fully resolved.
"We apologise for any inconvenience this may have caused."
It is not yet clear what caused the service disruption, or how widespread it was.
A number of users had earlier contacted the network on Twitter, saying they were unable to make calls and were also having problems receiving them.
At the time, 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 among their customers.
The incident came after a major outage in December saw millions of O2 customers 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 the result of a software issue.