Three customers have faced a day of disruption; with dropped calls, undelivered text messages and missing mobile internet.
The network provider said issues arose after a software upgrade failed - leaving hundreds of thousands experiencing what it described as "technical difficulties".
The company apologised for the issues, admitting a low-tech solution of turning devices off and back on again may help restore service to affected users.
"We're is currently experiencing technical difficulties with our service across voice, text and data," it tweeted.
Continuing, it said this "means that some customers will be experiencing an intermittent service."
“Our engineers are working on the issue now to fix the problem as soon as possible. We are sorry for the inconvenience caused to our customers.”
It later added most service users had had their connection restored.
As well as its mobile network, Three’s website was also unavailable on Thursday morning, instead displaying a message which said it was down for “essential maintenance”.
According to online service monitoring website DownDetector, the issues with Three began at around 11pm on Wednesday.
The majority of users who flagged the issue with DownDetector cited problems with mobile internet.
The incident came on the day another mobile operator – O2 – became the latest to begin the rollout of its next-generation 5G network.