Phone provider O2 has said its service is "back to normal" after customers complained of problems with reception in several parts of the UK.
The company has apologised after people in areas including London, Manchester, Bristol, Glasgow and parts of Northern Ireland said they had lost signal.
Business Secretary Sajid Javid was among those expressing frustration on Twitter, asking O2 to "sort it out".
O2 said network equipment was to blame for the temporary lack of reception.
A spokeswoman said "some customers experienced a problem accessing our network" on Monday evening and apologised to those affected before the service returned to normal at 23:35 BST.
She said: "The problem was caused by some network equipment that was temporarily unable to correctly process mobile phone traffic for some of our customers.
"To be more specific, it was primarily the system that allows overseas customers to roam on to our network.
"The problem also impacted a second piece of equipment that allows calls, texts and data for UK customers to be managed correctly on our network.
"Having identified this, we have now isolated the problem to prevent this from happening again."
Many O2 customers had taken to social media complaining of not being able to send or receive calls and text messages, as well as problems with the 3G internet signal.
While the problems were ongoing, it had advised people to reset their device, and visit its support page if problems persisted.
Some customers lost reception for up to five hours on Monday.
Media and technology analyst Matthew Howett told BBC Breakfast the fact that O2 had advised customers to turn their phones off and on again suggested "a problem with registering with the network".