The mobile network Three acknowledged the "technical difficulties of voice, text and data," leaving many customers unable to use their devices.
According to the Down Detector website, the problem seems to have started on Wednesday night.
At the end of Thursday, the company said it could restore the services of most of its subscribers.
Three people who have apologized for this issue said, "We are categorizing this now."
Above Was added on Thursday evening: "Most of our customers are back in the network. Engineers will keep working overnight on the remaining issues."
Previously, we informed our customers that the issue "turning the phone off and on or turning airplane mode on or off may solve the problem" still exists.
Three of them have about 10 million customers in the UK.
ID Mobile, a virtual network using Three's infrastructure, was also affected.
The nationwide problem began after maintenance work on Three's network infrastructure.
On Wednesday, the competition network O2 turned on next generation 5G services in several UK cities.
"Oi, did you separate our network so you could connect 5G? Not cool."
One customer said the joke would be "cute" if the problem didn't last for more than nine hours.
Too many customers tried to access the health check tool on Three's website, which was temporarily unavailable on Thursday morning.
The queuing system is turned on to restrict access to the tool.
Ben Wood, an analyst at CCS Insight consultants, said, “History shows that once services are restored, people quickly forget about this problem.
"The challenge for Third UK is to bring the network back online reliably. It can often take some time for things to stabilize after this massive power outage, which can lead to intermittent service for some time after the original problem."