Telecoms operator Vodafone has been fined £4.6m by Ofcom for “serious and sustained breaches of consumer protection rules”.
It follows an earlier investigation that found Vodafone had misled pay-as-you-go customers over top-up payments.
The regulator also said Vodafone had broken the rules on handling customer complaints.
Vodafone offered its “profound apologies” for the failures.
The fines stem from two investigations into Vodafone, which has 20 million mobile customers in the UK.
One found that 10,452 pay-as-you-go customers lost out when Vodafone failed to credit their accounts after they paid to top-up their mobile phone credit.
The affected customers collectively lost £150,000 over a 17-month period, Ofcom said.
“Vodafone also failed to act quickly enough to identify or address these problems,” it added.
A second investigation found that Vodafone had failed to handle customer complaints properly.
The firm’s customer service agents were not given “sufficiently clear guidance” on what constituted a complaint, while poor processes meant some complaints were not handled “in a fair, timely manner”.
In a statement, Vodafone blamed the issues under investigation on errors linked to a “complex IT migration” which involved more than 28.5 million of its customer accounts.
The company added it had “fully refunded or re-credited” 10,422 pay-as-you-go customers out of the 10,452 affected. It said it was unable to track down the remaining 30 affected.
“Everyone who works for us is expected to do their utmost to meet our customers’ needs,” Vodafone said.
“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.”