O2 has confirmed that one of Apple's most popular iPhones is not working as it should on the company's network.
The iPhone XR has completely lost its signal several times a day, some O2 customers say on Twitter.
Many customers were unable to make and receive calls or send and receive text messages.
They also struggled for a reliable 4G Internet connection by making redundant applications like Facebook and Instagram unless there is wifi.
"We are working closely with our partners to resolve an intermittent issue affecting some of our customers using the iPhone XR," an O2 spokeswoman told the BBC.
"We thank all customers affected by your patience."
O2 declined to say exactly how many users were affected by the problem and Apple did not respond to a comment request.
Turning the phone off and on again temporarily fixed the problem, O2 said.
O2 customer Jim Buckley told BBC News that he had noticed a network problem on December 16.
"I've had virtually no sign since then," he said.
"It may come with a weak signal for a few minutes once or twice a day, and then come back again."
Buckley said he turned to buying a second-hand iPhone 7 for £ 180 as his son had special needs and needed to contact him in case of an emergency at school.
He said O2 had said that Apple had introduced the problem with a recent iPhone iOS software update.
BBC News is waiting for O2 to confirm that this is the case.
Buckley added that O2 offered him a one month free line rental as compensation.
O2 and Apple are telling customers looking for answers on Twitter to direct them to the message.
Launched in October 2018, the iPhone XR costs £ 629 for the basic 64 GB version and £ 679 for the 128 GB version.
The device can be obtained through an O2 contract for £ 30 per month.