OFCOM is warning of a scam that could leave you with a £300 bill for a phone call you don't know you've made.
In the scam, customers receive calls from an 0845 or 0843 number, but ignore it.
Then weeks later their mobile phone bill shows they called that number back and racked up a huge bill for calls lasting between three and 12 hours.
Customers from Vodafone, EE and O2 customers have all reported the scam to Money Mail. Often they say they don't even remember missing the original call or ignored it because they didn't know the number.
No-one seems to know how return calls are being made.
An Ofcom spokesman said: "We are very concerned to hear of a number of people receiving mobile charges they didn’t expect.
"Ofcom is working with the mobile operators, industry experts and partner regulators to establish the causes and address the problem.
"We are pleased that Vodafone is blocking suspicious numbers and refunding affected customers. We advise those who believe they are affected to contact their phone company promptly."
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