Three UK has confirmed to DCD that a technical issue at one of the company's data centers was to blame for a network outage last Friday (December 1).
The operator has apologized for the outage, which affected thousands of its users last week.
It impacted more than 20,000 users, with customers complaining that they were unable to send texts, make calls, or use mobile Internet services.
"Following yesterday’s network issue, services have now been fully restored," said Three on Saturday (December 2), noting it only impacted a small number of its users.
"If any customer is still having an issue, they should fully restart their device to reconnect to our network. We apologize for any inconvenience caused."
The outage also affected Smarty, an MVNO which uses Three's network.
Three is poised to merge with Vodafone in the UK in a deal worth £15 billion ($19bn), to create the country's biggest mobile operator, with 27 million combined subscribers.
In September, Vodafone and EE suffered an outage, which only affected calls and texts sent between their users. Vodafone blamed the issue on EE.