O2 says it is investigating the cause of a technical problem that led to millions of its customers suffering connection issues on Friday.
The firm has apologised after after around 10% of its 23 million customers were affected by the outage.
O2 admitted people in some areas had been unable to make or receive calls, texts or access data services.
On Friday night, an O2 spokesperson told Sky News: "Further to our previous update where some customers in some areas were unable to make or receive calls, send texts or use data, service has been fully restored for all remaining impacted customers.
"Once again we're sorry for any inconvenience this has caused."
Asked about whether any compensation would be offered, the spokesperson said: ''Our first priority was to get customers back up and in service. We'll now need to conduct a full investigation into the cause of the problem.''
The problem began when one of O2's network nodes, which help connect and manage traffic, failed at lunchtime.
O2 issued a statement saying the fault had been identified and fixed on Friday afternoon, but admitted the problems were still being experienced into the evening.
The company said it is trying to discover the cause of the glitch, but said it was the not the same fault that led to widespread connection issues in July, when some customers had a 24-hour service outage.
The company had only just recovered from a separate problem that began on Thursday, involving people unable to make calls or use data with some 2G phones.
O2 said it had had a problem with its 2G service in northwest Scotland, and anyone with a 2G phone would not be able to use it for calls, texts, email, or the internet.
Its 3G service had still been working and the 2G problem was marked up as "fixed" at 9.25am on Friday.
As services returned to normal, chief executive Ronan Dunne told Sky News he was "embarrassed" by the network fault and worried about the potential for a backlash.
He blamed the disruption on a "piece of network hardware which manages the registration of handsets" and insisted such an issue was extremely rare.
Disgruntled users took to social media to discuss the latest outage, with one Twitter user saying: "Got 02 problems. I got 99 contacts and I can't text one."