BT accounted for 5% of the cold calls received by 754 people who made a note of the calls they received in a two-week period.
However, only 65% of those surveyed received any cold calls, and only 15% received more than five. Which? concluded that the Telephone Preference Service worked, with those registered receiving half as many as those who were not registered.
Coming joint second to BT with 2% were 3, Anglian Home Improvements, British Gas, Homecall, OneTel and T-Mobile.
A BT spokesman told the magazine: "We won't call anyone who asks us not to and we explicitly give customers the option not to take further calls from us when they sign up to BT Privacy."
By company sector, telecoms made 30% of cold calls, followed by financial companies on 18%, home improvement companies on 15%, then gas and electricity companies on 5% and charities on 4%.
Which? defined cold calls as unsolicited calls from companies trying to sell something, offer a service, get someone to take part in a competition, and from charities making fundraising calls. It included unknown companies and companies with which recipients were already customers.
Which? also carried out a doorstep cold calling survey, which identified British Gas as making the most cold calls with 6% of the total.
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