Three has apologised after thousands of people reported they were unable to make phone calls on the UK mobile network.
Downdetector, which tracks website outages, showed more than 10,000 people reported issues with Three in the UK on Thursday.
There have also been several thousand reports from users of Smarty and ID Mobile – smaller mobile companies which use Three’s network.
“We’re aware of an issue affecting a small percentage of voice services, our team are working hard to fix this as soon as possible,” Three said in a post on X.
“We’re really sorry for any inconvenience caused.”
The firm said people were still able to use mobile data services and make 999 calls during the outage.
Three’s support team has been telling customers that it does not “have a timeframe” for a fix, but that the firm is “working hard to resolve this as soon as possible”.
Three has around 10.5m customers across the UK, according to its website, but it is unclear how many of them are affected by the outages.
Many people on social media have shared their frustration at the outage and described the disruption they said it had caused them.
One person claimed they had “missed a medical appointment” as a result of being unable to receive calls, while another said the issues had left their daughter “stranded”.
And several people have claimed they would be leaving the network altogether.
It is not known whether customers will be able to claim compensation for the outage, although according to the website of telecoms regulator Ofcom, it “may be appropriate” for providers to offer refunds “while repairs are being made”.
It comes a month after the UK regulator gave the go-ahead for Three to merge with former rival Vodafone in a £16.5bn deal.
It comes the same day a major outage affected artificial intelligence tool ChatGPT.
