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* Company was slow to address problem - Ofcom
* More than 10,000 customers affected
* Vodafone says determined to put things right (adds details on Vodafone customer complaints)
LONDON, Oct 26 (Reuters) - Britain has fined Vodafone a record 4.6 million pounds ($5.60 million) for "seriousand sustained" customer failures, including not updatingaccounts when mobile phone users topped up their credit to makecalls.
Vodafone, the world's second-largest mobile operator, alsofailed to act quickly enough to identify or address theproblems, the regulator said, which stemmed from a move to a newbilling system.
Some 10,452 of the mobile phone giant's pay-as-you-gocustomers collectively lost 150,000 pounds over a 17-monthperiod between the end of 2013 and April 2015.
Vodafone said it deeply regretted the system and processfailures and had refunded the vast majority of the affectedcustomers.
Ofcom Consumer Group Director Lindsey Fussell said thefailings were "serious and unacceptable" and the fines, whichare the highest ever imposed by the regulator, sent a clearwarning to all telecoms companies.
"Phone services are a vital part of people's lives, and weexpect all customers to be treated fairly and in good faith,"she said on Wednesday.
The new billing system led to a sharp rise in customercomplaints in Vodafone's home market, where it has about 19.5million customers.
It has been the worst performing network in terms ofcustomer service since the final quarter of 2014, according toOfcom data, with complaints spiking at 32 per 100,000 customersin the final quarter of 2015, more than three times the industryaverage.
Vodafone Chief Executive Vittorio Colao apologised tocustomers in May for the billing problems.
The company said on Wednesday that all but 30 customers hadbeen fully refunded or re-credited, with an average refund of14.35 pounds, and it donated 100,000 pounds to charity to ensureit did not profit from the 30 customers it could not track down.
"This has been an unhappy episode for all of us at Vodafone:we know we let our customers down," the company said. "We aredetermined to put everything right."
Vodafone said it was confident its customers were alreadybeginning to see the benefits of its investment in its newsystems.
Britain's regulators have stepped up the pressure in recentyears to make sure customers are being protected. Ofcom said inSeptember that Sky, Britain's biggest pay-TV group, mayhave violated consumer rules by making it too difficult forcustomers to cancel or switch providers.
($1 = 0.8220 pounds) (Reporting by Paul Sandle; editing by Kate Holton and SusanThomas)
