UK mobile phone users are often paying too much because staff at some main operators are unable to answer tariff questions, a consumer report says.
The UK mobile phone market is seen as being completely saturated
Staff at O2 were the most likely not to be able to answer a tariff question, according to a survey of the UK's six biggest mobile firms by Onecompare.com.
Virgin Mobile staff were the most accurate, followed by workers at 3, T-Mobile, Orange and Vodafone.
O2 said that the survey was not broad enough to draw an accurate conclusion.
Onecompare.com used a mystery shopper throughout January and February to ask questions about peak and off-peak rates, and international roaming costs.
Each mobile phone provider was called 100 times.
"We do not think that Onecompare are right to draw their conclusions based on a sample of just 100 calls," O2 said, adding it was surprised by the results.
"We receive, on average, 150,000 calls a day, which means we received over eight million calls in January and February of this year," the company said.
An O2 spokesperson said the company would study the results "as it contradicts the independent recognition we have been receiving for the quality of customer service we deliver".
"We continue to invest heavily in making sure that our customers receive the best possible service, which includes the recent opening of our fourth UK customer services centre in Glasgow," O2 said.
In its survey, Onecompare said that O2 customers were the most likely to be penalised with higher bills because wrong or inadequate price information had been given by staff in four out of 10 cases.
According to the survey, Vodafone staff answered 20% of questions incorrectly, while Orange customer service got 15.4% wrong when asked about the cost of a basic domestic calls and overseas charges.
Virgin came top of the class, with its customer care staff making an error 8.2% of the time, compared with 9.8% at rival 3.
In a separate survey of 1,842 mobile phone users in the first two months of this year, Onecompare found 49% of people questioned were not aware that it would cost them money to receive calls abroad.
Half of those questioned were unhappy with the customer service provided by their mobile phone company.