Mobile network Three has acknowledged it is experiencing "technical difficulties with voice, text and data", leaving many customers offline.
The problems appear to have started on Wednesday evening, according to the Down Detector website.
Customers across the UK have taken to social media to complain about the loss of service.
Three apologised for the problem and said it was "sorting this out right now".
So many customers tried to access the status checker on Three's website that it was temporarily unavailable on Thursday morning.
A queuing system has been switched on, to limit access to the tool.