EE, the UK's largest mobile phone operator, has been fined £1m by the regulator Ofcom for breaching rules on handling customer complaints.
Ofcom said from 2011 to 2014, the firm did not provide its customers with full information about their right to take complaints to an independent body.
EE should have informed customers about this in writing, but did not, it found.
The investigation into EE is part of Ofcom's wider look at complaints handling by telecoms companies.
EE stressed that the fine related to historic conduct.
"While this in no way excuses it, it is important to note that we identified issues in our complaints handling and began our programme to tackle these problems head-on in 2013, before Ofcom started their investigation," a spokesperson for the company said.
"We have made considerable improvements since then. Ofcom's current figures highlight that complaints into Ofcom about EE have fallen by 50% in the past year."
Ofcom point that the 50% figure relates solely to complaints about mobile services, but that complaints about EE broadband and landlines services have actually risen by 50% and 30% respectively.
EE customers have the right to take complaints that cannot be resolved to an independent body after two months have elapsed from the time of the original complaint or after an official "deadlock" letter has been received.
Between 22 July 2011 and 8 April 2014, Ofcom found that a number of customers who had requested a "deadlock letter" as a precursor to referring their complaint to the independent body never received such a letter.
It also found that EE did not notify some customers on their paper bills that they could refer any complaint to this body free of charge.
"It's vital that customers can access all the information they need when they're pursuing a complaint," said Ofcom's Claudio Pollack.
"Ofcom imposes strict rules on how providers must handle complaints and treats any breach of these rules very seriously. The fine imposed against EE takes account of the serious failings that occurred in the company's complaints handling, and the extended period over which these took place."
The regulator said EE had 20 days to pay the penalty, which would then be passed on the UK Treasury.