BT has topped a quarterly complaints table for broadband and pay TV, while BT-owned Plusnet was the most complained-about landline provider, Ofcom figures show.
Plusnet, which was recently fined £880,000 for continuing to charge more than 1,000 former landline and broadband customers, drew 24 complaints for every 100,000 landline customers between October and December, ahead of Post Office and TalkTalk on 23 complaints.
Sky drew the fewest complaints, at seven per 100,000 customers.
BT attracted the most complaints among broadband providers with 33 per 100,000 customers, followed by Plusnet with 30 and TalkTalk with 29.
Sky was again the best performer, with eight complaints per 100,000 accounts.
BT fared particularly poorly for pay TV complaints, attracting 17 per 100,000 customers, well ahead of TalkTalk and Virgin Media, both on nine, and significantly ahead of best performer Sky on one.
Vodafone was the most complained-about mobile provider, on 24 complaints, followed by Talk Mobile (nine) and TalkTalk and Virgin Mobile, both on eight.
Tesco was the best performer among mobile providers, attracting less than 0.5 complaints per 100,000 customers.
The total number of complaints made to Ofcom increased between the third and fourth quarters of last year.
Lindsey Fussell, Ofcom's Consumer Group Director, said: "Providers must get on and deliver consistently excellent customer service, and we expect this to be their number one priority.
"When companies get things wrong, we won't hesitate to investigate and potentially levy fines."
BT said: "We apologise to customers we've let down.
"BT cares about the service it gives and we are investing heavily to make improvements. We're on track with our plan for 90% of our customers' calls to be answered by advisers in the UK and Ireland, which involves recruiting more than 1,500 people.
"We've invested in a major upgrade in the software our call centre advisers use, which is also enabling more customers to serve themselves online.
"Customers will see an improvement as these initiatives take effect. We intend to start publishing our key customer service improvements on our website so customers can track the progress for themselves."