Mobile network Three has acknowledged it is experiencing "technical difficulties with voice, text and data", leaving many customers offline.
The problems appear to have started on Wednesday evening, according to the Down Detector website.
Customers across the UK have taken to social media to complain about the loss of service.
Three apologised for the problem and said it was "sorting this out right now".
"Our engineers are working to fix the issue and the service is returning to normal, and we expect it to be resolved over the course of the day.
"We advise our customers to turn their phones off and on or turn airplane mode on and off, which may resolve the issue," it said in a statement.
The problems, which are nationwide, started after some maintenance work on Three's network infrastructure.
It is not sure how many of its 10 million customers are affected.
ID Mobile, a virtual network that uses Three's infrastructure, is also experiencing problems.
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. lobbyistdlive
A queuing system has been switched on, to limit access to the tool. Dlive
History shows that once service is restored people quickly forget about the issues," said Ben Wood, an analyst at the CCS Insight consultancy. pewdiepiedlive
"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.