Vodafone customers have been urged to check their bills immediately after thousands of customers reported errors.
Martin Lewis, founder of Moneysavingexpert.com, said customers should check that their direct debit had been set up correctly and that they were on the right tariff after his site received a huge number of reports of these issues and poor customer service.
"This is a clarion call as one of Britain’s biggest companies, revealed as its most valuable brand, has frankly cocked-up big time," Lewis said.
“The thousands of complaints already received about Vodafone are likely to be a drop in the ocean compared with the amount of people likely affected, as most people don’t rigorously check their bills or tariffs," he added.
Kimberley contacted MoneySavingExpert.com to say that she had experienced billing issues for six months by the time she was charged £50 for a number she doesn't own.
Another customer called Anna told the site: "I was overcharged every month for four months and had to call four times each time to get it sorted. Will be leaving them when I can!"
The problems started to occur when Vodafone moved customers from its legacy system to a new state-of-the-art system, the company said.
Vodafone said it needed to move customer details to the new platform to give them greater control and visibility over their whole account, including real-time access to their bills. The new system also gives Vodafone customer services personel access to one set of customer information.
"This was always going to be a highly ambitious and complex programme but the impact of running an IT and a Contact Centre transformation in parallel was underestimated: the combination of new systems, new processes and new customer services agents impacted service levels significantly," Vodafone said.
Ofcom has received more than three times as many complaints about Vodafone than any other network this year. Pay-monthly complaints about Vodafone have more than doubled from from 14 per 100,000 customers in the first three months of 2015 to 32 per 100,000 in the final three months.
Vodafone customers are urged to check their bill for errors, paying special attention to whether they are on the right tariff and any extra charges.
They should also check bank statements to make sure they are not being charged more than once.
Vodafone said it had hired more customer service staff to handle calls from affected customers.
Customers who have discovered errors can reach Vodafone by calling 191 free from a Vodafone mobile or through the the help pages online.
