Mobile network Three has acknowledged “technical difficulties with voice, text and data” that left many customers unable to use their devices.
The problems appear to have started on Wednesday night, according to the Down Detector website.
By late Thursday, the company said it had managed to restore service for most of its subscribers.
Three apologised for the problem and said it was “sorting this out right now”.
On Thursday evening it added: “Most of our customers are now back on our network. our engineers will continue to work into the night on any remaining issues.”
Earlier it had advised customers still experiencing problems “to turn their phones off and on or turn airplane mode on and off, which may resolve the issue”.
Three has about 10 million customers in the UK.
ID Mobile, a virtual network that uses Three’s infrastructure, was also affected.
The problems, which were nationwide, started after some maintenance work on Three’s network infrastructure.
On Wednesday, rival network O2 switched on its next-generation 5G service in a number of UK cities.
Three tagged O2 in a tweet saying: “Oi, did you unplug our network so you could plug in your 5G? not cool guys.”
One customer said the joke would have been “cute” if the problems had not been ongoing for more than nine hours.
So many customers tried to access the status checker on Three’s website that it was temporarily unavailable on Thursday morning.
A queuing system was switched on, to limit access to the tool.
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“History shows that once service is restored people quickly forget about the issues,” said Ben Wood, an analyst at the CCS Insight consultancy.
“The challenge for Three UK will be getting its network back online reliably. Often it can take time for things to stabilise after such a massive outage, which can lead to intermittent service for a period of time after the original problems.”