TalkTalk has insisted the latest complaints figures from Ofcom do not reflect the current customer experience.
According to the regulator, TalkTalk was the subject of 29 complaints per 100,000 customers during the first quarter of 2018.
Responding to the figures, a spokeswoman acknowledged they are "disappointing", but said these "historical numbers" do not accurately portray the current situation.
"The most recent data shows a significant reduction in complaint numbers, customer satisfaction is on the rise and we've seen more customers than ever staying with us this year," she said.
BT also performed poorly in Ofcom's figures, as it was the subject of 23 complaints per 100,000 customers.
"We know we can do better for our customers and we've made a commitment to answer all our customer calls in the UK and Ireland by 2020," a spokesman commented.
The representative also pointed out that BT is now a much more visible presence on UK high streets, while it has launched a completely redesigned, personalised customer experience for 24/7 help and advice on BT.com.