Three Mobile has apologised after customers were not able to send texts or make phone calls.
The company said it experienced a "temporary works issue" which affected its service during Saturday afternoon and evening.
It said that although calls have been restored they are working to restore a full service.
In a statement, Three said: "We have experienced a temporary network issue which has affected calls and texts during Saturday afternoon and evening.
"Calls have been restored and we are working to restore full service. We are currently investigating the cause of the service disruption. We apologise sincerely for any inconvenience caused."
Some customers reported that they had received misdirected texts or texts from unknown numbers.
The statement added: "We are investigating these incidences but if any customer is concerned about this they should contact Three customer services."
Customers are advised to "ignore all text messages that you deem incorrect".
The company was hit by a data breach in 2016 which saw personal details, including names and addresses, accessed unlawfully.