Communications company EE has been fined £1 million for "serious failings" in handling customer complaints, Ofcom said.
EE failed to adequately inform customers they were entitled to take their complaint to an independent body for free if it remained unresolved after eight weeks.
Unhappy customers were also not made aware that they could take the complaint to an alternative dispute resolution (ADR) if they reached a stalemate or 'deadlock' with the company before eight weeks.
ADR schemes allow customers to refer complaints for free to an independent body which can reach an impartial judgment, Ofcom said.
EE's failure to comply with industry regulations impacted on mobile phone, broadband and landline customers.
Claudio Pollack, Ofcom's consumer and content group director, said: "It's vital that customers can access all the information they need when they're pursuing a complaint.
"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."
Complaints about EE mobile phone services received by Ofcom have fallen by 50% in the past year, but broadband and landline complaints have rapidly risen, by 50% and 30% respectively.
Ofcom said EE has since amended its customer complaints code to include correct references about steps customers can take to resolve a complaint.
The investigation into EE was carried out between July 2011 and April 2014.
An EE spokeswoman said: "This fine relates to our historic performance regarding complaints handling, collected from 2011 to April 2014.
"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.
"We have made considerable improvements since then."
Which? executive director Richard Lloyd said: "Telecoms are now an essential part of people's lives, so it's disappointing customer complaints haven't been dealt with fairly.
"Companies won't always get it right and that's why it's important people feel protected and empowered to escalate their complaint to get their issue resolved."
The £1 million penalty is payable to Ofcom within 20 days and then is passed on to the Treasury, Ofcom said.
Steve Nowottny, consumer editor at MoneySavingExpert.com said: "It's great to see Ofcom take a tough stance on companies which fall short – many people simply won't know they have the right to appeal, so it's important they're told about it.
"It's also worth noting that while EE insists the problems it's been penalised for have long since been resolved, many customers are still unhappy with its customer service today.
"In MoneySavingExpert's poll last month it took the wooden spoon for home phone customer service, while 40% rated its broadband customer service as poor."