Hundreds of thousands of O2 customers were left without mobile phone access for several hours on Thursday after the operator's network crashed.
Engineers worked through the night to fix the fault but many mobile users were left facing a second day of being unable to make or receive calls.
Just before 2pm O2 said it has restored service to its affected customers, but many were still furious that they had been left without access for so long.
"Following previous updates, our tests now show that all our 2G and 3G services have been fully restored for affected customers. If any customers are still having problems we recommend they turn their phone off and on again.
"Once again, we are sorry."
Network problems started to be reported early on Wednesday afternoon, but it is not yet know how many of O2's 23 million UK customers were affected.
O2 in the UK
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Many disgruntled customers took to Twitter this morning to vent their frustration at still being unable to use their phones.
Sean Foster (@SeanFoster) tweeted: "Arrrrrrrghhhhh! My phone was fine all day yesterday and now after their failed overnight work o2 have screwed it!!"
Another Twitter user Leanna May (@LeannaMai) said: "I can't bare this any longer!! THREE @o2 contracts, NONE WORK. And I'm at home pregnant in agony! Great, who should I call? No one!!"
David de la Mere (@dmeeno) also took to the micro blogging site, tweeting: "I probably shouldn't have been so smug about having an @O2 signal yesterday. I don't have one now."
O2, which was formed in 2001, following the demerger from British Telecom of its former mobile business, BT Wireless, is urging customers to check their Twitter and Facebook feeds for updates.
Tesco, which uses O2's masts for its Tesco Mobile network, confirmed that some of its customers were being affected this morning.
"We are currently seeing a problem on our network affecting some of our customers," a spokeswoman said.
"There is an issue within part of O2's core network that is preventing some mobile phones from successfully connecting.
"We're sorry about the inconvenience this may cause our customers, and O2 engineers are dealing with the problem to restore full service as soon as possible."