O2 phone customers could still not access data services such as the internet, or make calls, on Thursday after the network crashed.
Overnight, engineers attempted to fix the problem, but many phone users will not be able to communicate via their O2 phones for a second day.
Customers have had issues with calls, text messages and data services, including internet access on smartphones.
The first reports of problems with the network were seen on Wednesday afternoon. Out of O2's 23 million customers, it is currently unknown how many have had problems.
A spokeswoman for the company said: "We, and our central supplier, have deployed all possible resources and have been working through the night to restore service as soon as possible."
According to the spokeswoman, the number of people affected by the nationwide network issues is unclear, but it is estimated "thousands" have had problems.
O2 took to Twitter to speak to customers, writing: "Our engineers are doing their level best to resolve the current network issue. Apologies for the inconvenience caused."
It is not known when the repairs will be complete.