BT, TalkTalk and Scottish Power operate the UK's most dreaded call centres, a survey has found.
More than 7,000 consumers ranked the three companies bottom of a satisfaction survey based on staff knowledge, phone menu system, politeness, helpfulness and waiting times, Which? said.
Consumers are most frustrated with broadband and energy providers, who ranked the worst overall for their long waiting times, poor staff knowledge and lengthy phone menus.
BT, TalkTalk and Scottish Power all scored an overall rating of just one star.
The poll found only just over half of broadband providers (58%) answered calls in the first five minutes, compared with 89% of car insurers and 83% of banks.
Around one in 10 customers (12%) felt the person they spoke to at their energy provider had poor or very poor product knowledge, a belief shared by 17% of callers to broadband companies.
A third of BT Broadband customers (34%) rated its customer service as poor or very poor, while 21% of Scottish Power electricity customers had spent more than 20 minutes waiting for an answer in the last six months.
Almost a fifth (18%) of npower's electricity customers thought staff knowledge of its products and services was poor or very poor.
Almost all those surveyed (95%) said calls should be answered within five minutes by a human voice, not an automated service, and 90% said UK companies should operate UK call centres.
Which? executive director Richard Lloyd said: "Unfortunately, poor customer service from call centres has become a 21st century bug-bear for too many people.
"Customers should vote with their feet if they're tired of waiting or fed up with the service they get.
"We want the worst offenders in our survey to raise their game by answering the phone quicker and improving staff training to demonstrate they really value their customers' time."
:: Which? surveyed 7,057 members between February 26 and March 13.
BT said: "We're disappointed by the results of the survey. We are investing significantly in our systems, processes and people to improve our customer service in order to make our service easier for our customers.
"We offer customers a range of ways to contact us so they can use the method they prefer. It's free to contact us by phone and we've rolled out programmes to solve problems first time and shorten waiting times. Our online help and live chat gives customers instant access to experts when they need them.
"Over the last year, we have taken on hundreds of new call UK centre staff and have also set up a dedicated team for complex complaints so customers have a person who owns their issue until it's resolved."
A TalkTalk spokesman said: "We're absolutely committed to improving customer service. We've launched an extensive programme to enhance agents' training, simplify our systems to resolve faults more efficiently and make it easier for customers to manage their account online.
"We've made significant improvements to levels of customer satisfaction, but recognise there's always more to be done."
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