By Anna Pollitt
A sophisticated new mobile phone scam is charging customers hundreds of pounds for calls they don't make.
Victims are being hit with huge bills after being contacted by numbers beginning in 0845 or 0843.
The scam affects some people whether they answer the calls, ignore them, or call the number back, an investigation by Money Mail has found.
Customers complained after receiving enormous bills with itemised logs showing outgoing calls lasting up to 12 hours to premium rate numbers.
Ofcom has confirmed it is an industry-wide problem affecting customers on several networks.
One theory is that criminals are buying the premium numbers, setting fees high and then hacking customers' phones.
Vodafone customer Ruth Dance, from Bracknell, Berkshire, was billed £300 after being called from 0843 9800 146.
Another customer, Diana Dentith, from Lowestoft, in Suffolk, told the paper her usual monthly bill of £9 shot up to £375 after she missed a call from 0845 429 0015.
When customers answer or call back the numbers, they typically connect to recorded messages purporting to be claims management companies, covering areas such as accident and injury, and payment protection insurance (PPI).
Have you been affected by the scam? Email email@example.com
She said that Vodafone initially told her that she must have called the number for 12 hours.
The firm told ITV News it has now reimbursed "anyone that we know about" and said that customers who fear they have been targeted should dial 191.
It said it is working with regulator Ofcom and other mobile operators "to identify and close down this issue as soon as possible."
EE customers have also complained of being hit by charges for calls they did not make.
The firm has been contacted for comment.
Telecommunications expert Ben Levitan said: "There are people who spend their lives looking at phone companies' systems and ways to make money from them.
"People share these secrets online and use them, but the criminals can be very hard to trace and catch."
Ofcom said it is "very concerned" by the scam and said it is working with industry experts and mobile operators.
Customers can use the "Block this caller" function on their phone if they have been contacted by any unwanted premium rate numbers.
Paul Moore, a computer security consultant, told ITV News that while it's possible to block the number, "it almost certainly wouldn't prevent this abuse continuing."