Vodafone UK customers have been urged to check their bills following thousands of problems reported to consumer and complaints organisations.
MoneySavingExpert.com said it believed there were "potentially systemic failings" at Vodafone UK that suggested all of its nearly 20 million customers should be checking their bills.
It said it had been flooded with concerns including incorrectly set up direct debits, customers being put on the wrong tariff and poor customer service.
Customers also reported their accounts not being transferred to the new billing system, incorrect charges and charges following cancellation, and multiple payments being taken from accounts in a single month.
Martin Lewis, founder of MoneySavingExpert.com, said: "Yet it's likely there are many more who haven't noticed a whole host of issues - some have even found their credit score's been wrongly hit - so if you're a Vodafone UK customer our clarion call is you check your bill now."
The Communications Ombudsman, currently running a dedicated touch phone option for consumers calling about problems with Vodafone UK, said it saw "an increasing number of complaints over the last nine months", mainly over billing problems but also regarding customer service.
Around 60% of Vodafone UK complaints escalated to the Ombudsman over the past six months were upheld.
Complaints website Resolver said Vodafone attracted the most complaints of any telecoms firm in the last year, with 6,213 cases opened against it between May last year and April.
Vodafone UK also became the most complained-about pay-monthly mobile provider according to Ofcom figures released in March.
Complaints surged to 32 per 100,000 customers in the final three months of 2015, up from 20 in the previous quarter.
It comes after Vodafone UK moved customers over to a new billing system at the end of last year.
Ofcom said complaints received about Vodafone related to billing, pricing and charges, as well as complaints handling and fault, service or provision issues.
Vodafone UK told MoneySavingExpert: "We would like to apologise to any customers who have been affected by our recent customer service issues.
"Our teams, at all levels, want to provide our customers with a great service every time but we are clearly not there yet so are investing significant resources to do so as quickly as we can.
"Many of the recent issues relate to the move of our legacy billing and services platforms into one state-of-the-art system.
"This is to give customers far greater control and visibility of their whole account, including real-time billing access, while allowing our customer services' channels to have access to one set of customer information and treat customers as people not telephone numbers.
"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."
A spokeswoman added that Vodafone UK's own analysis of customer service feedback showed an improvement of 40 percentage points since the end of December.