Failure attracts attention of Business Secretary Sajid Javid.
O2‘s network suffered outages on Monday evening.
The network failures affected customers in London, Manchester, Leicester, Bristol, Sheffield, Birmingham, Glasgow, Leeds and parts of Northern Ireland.
Statistics collected by Downdetector saw complaints of outages peak at 2960 just before 9 PM, having started to rise at around 4 PM.
Disgruntled customers took to Twitter to express their ire, with O2 advising many customers directly to "reboot" their devices.
This included Secretary of State for Business and former culture secretary Sajid Javid, who wrote: "No signal @O2. Please sort it out".
"We’re aware of isolated instances where some customers have intermittent access to our service. We’re investigating and will update ASAP," O2 announced on Twitter.
Later, O2 indicated that the problem had been resolved, tweeting: "We’ve minimised the intermittent network issue reported earlier and some customers have seen their service return. We continue to monitor."
O2 has suffered notable outages before in 2012 and 2013.
The Telefónica-owned operator is the UK’s second-biggest mobile operator, behind EE. The upcoming purchase of O2 by Hutchinson Whampoa will see its network consolidated with that of the smaller Three, with the new conglomerate leapfrogging EE to take the top spot.