O2 has confirmed in a tweet that one of its third-party suppliers has identified a software issue in its system, which appears to be the cause of the fault. They’re currently working hard to resolve the issue so we would imagine it won’t be too long before its back up.
You can have a look at the O2 Status Tracker which will let you know the availability of the network in real time.
O2 have advised its customers to attempt to use WiFi where possible and have tweeted an unreserved apology.
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