Apple and O2 have confirmed that one of the most popular iPhones is not working as it should on O2's network.
The iPhone XR completely lost signal several times a day, some O2 customers said on Twitter.
Many customers have been unable to make and receive calls or send and receive texts.
They have also struggled to get a reliable 4G internet connection, making apps such as Facebook and Instagram redundant unless there is wi-fi.
"We're working closely with our partners to resolve an intermittent issue affecting some of our customers using iPhone XR," an O2 spokeswoman told the BBC.
"We thank any customers affected for their patience."
O2 declined to say exactly how many users were affected by the issue.
Turning the phone off and on again temporarily fixed the problem, O2 said.
Apple said the issue will be resolved in a new software release.
"We are aware of an issue causing intermittent network connectivity effecting some O2 customers, and we will have a fix in an upcoming software release," the company said.
O2 customer Jim Buckley told BBC News he had first noticed a network issue on 16 December.
"I've had virtually no signal at all since," he said.
"It may come with a weak signal for a few minutes once or twice a day then go again."
Mr Buckley said he had resorted to buying a second-hand iPhone 7 for £180 as his son had special needs and he needed to be contactable in case of an emergency at school.
He said O2 had told him Apple had introduced the issue with a recent update to the iPhone's iOS software.
BBC News is waiting for O2 to confirm if this is the case.
Mr Buckley added O2 had offered him a month's free line rental as compensation.
O2 and Apple are telling customers seeking answers on Twitter to direct message them.
@O2 and @AppleSupport when are you going to publicly update us, the paying customers with iPhone XR about this diabolical issue with constant loss of signal? Not via DM. Publicly acknowledge and respond!!!— Yetunde O (@MissWhyOh) December 30, 2019
Released in October 2018, the iPhone XR costs £629 for the basic 64GB version and £679 for the 128GB version.
The device can be obtained through an O2 contract for £30 a month.