Mobile phone 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 allows customers to refer complaints for free to an independent body which can reach an impartial judgment, Ofcom said.
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."
The investigation into EE was carried out between July 2011 and April 2014.
Ofcom said EE has since amended its Customer Complaints Code to include correct references about steps customers can take to resolve a complaint.
The £1 million penalty is payable to Ofcom within 20 days and then passed on to the Treasury, Ofcom said.