Customers on O2's mobile phone network experienced widespread outages on Friday.
Many reported problems downloading data and making calls on their devices, across a broad swathe of the UK.
About 10% of customers were affected, O2 said.
"Approximately 10% of our customers have experienced intermittent service when making calls or using data since lunchtime," it told the Huffington Post UK in a statement on Friday.
"Some customers will already have had their service restored and we will have a full fix this afternoon. We are sorry for the inconvenience this has caused those customers."
The news comes after a week in which its customer service Twitter account won plaudits for - among other things - talking to its users like human beings.
Details on what exactly is the issues were slight but on Twitter users were quick to vent their frustration.
The outage is reminiscent of a similar issue in July when the network saw 24 hours of outages and blackouts.
O2 eventually had to offer affected customers compensation as a result of the problem.