UK mobile operator Three faces questions from communications regulator Ofcom, following a data services outage that left many customers without 3G internet access for long periods on Tuesday.
The network has issued an apology for the outage but, in a statement to Sky News, Ofcom said it will seek clarification over the incident.
A spokesperson for the regulator said: "Ofcom is aware that Three customers have been affected by a recent mobile data outage on its network.
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"We understand that data services have now been restored but are continuing to liaise with Three to understand the cause of problem."
All night long
Three was quick to say sorry to its customers on Wednesday, claiming its engineers pulled an all-nighter to resolve the problems, but the official communications failed to offer any insight as to the cause.
"Our engineers have worked through the night to resolve some issues related to yesterday's internet access problems that affected some of our customers' internet access.
"All services, including My3 account services have been restored and we are monitoring closely so we can address any performance issues if they arise.
"Voice and text services remained available throughout. We apologise for any interruption of service and for those affected - thank you for your patience."
If in doubt, reboot
The network had claimed on Tuesday afternoon that the issue had been fixed, but users continued to experience problems well into the evening.
Any of the Three's 9 million customers who're still without data service have been advised to reboot their handset.