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Virgin Mobile customers across the UK have been struggling to make calls, send text messages and use mobile data.
Since this morning there have been complaints in London, the Midlands, the North West, Bristol and Scotland.
Customers vented their fury on Twitter, with some complaining they had been left in the dark.
Virgin Mobile told the BBC it was not disclosing the number of affected customers.
A spokesman said: "We're working quickly to resolve an issue that's affecting some of our customers making calls, sending text messages and using mobile data.
"We apologise to our customers for the inconvenience caused."
According to Downdetector, which tracks mentions of mobile outages on social media, complaints began en masse at around 11:00 on Tuesday.
It said most comments had been about mobile phone services, followed by mobile internet. A minority mentioned issues with their Virgin cable internet services.
One customer tweeted: "@virginmedia why are you posting trivial polls and ads on your site, but nothing about the mass outage on mobiles? Incredibly frustrating and very poor comms."
Another said: "I'm a full-time carer for my disabled mother. We're both on Virgin Mobile. You've crippled my day with this outage."
According to Ofcom, the level of satisfaction with mobile providers is generally high across the UK.
However, according to its most recent customer satisfaction survey, Virgin Mobile is the most complained about of the major telecoms companies.