Vodafone has been slapped with a £4.6 million fine by Britain's telecom watchdog for breaching consumer protection rules.
The fine is a result of two investigations by Ofcom which found Vodafone was mishandling customer complaints and failed to credit the accounts of more than 10,452 pay-as-you-go customers who topped up their accounts.
The breaches took place between January 2014 and November 2015, and December 2013 and April 2015, respectively.
Ofcom consumer group director Lindsey Fussell said: "Vodafone's failings were serious and unacceptable, and these fines send a clear warning to all telecoms companies."
The fine must be paid to Ofcom within 20 working days, and the money will be passed to the Treasury.
Vodafone said: "We deeply regret these system and process failures. We are completely focused on serving our customers: everyone who works for us is expected to do their utmost to meet our customers' needs, day after day, and act quickly and efficiently if something goes wrong.
"It is clear from Ofcom's findings that we did not do that often enough or well enough on a number of occasions. We offer our profound apologies to anyone affected by these errors."
The company said it has fully refunded or recredited more than 10,422 customers an average of £14.35 each, and has made a £100,000 donation to "a number of UK charities" in lieu of being able to track down the remaining affected customers.