Vodafone, one of the world's largest mobile phone groups, has revealed the existence of secret wires that allow government agencies to listen to all conversations on its networks.
It said they were widely used in some of the 29 countries in which it operates in Europe and beyond.
The company outlined the details in a report that is described as the first of its kind. At 40,000 words, it is the most comprehensive survey yet of how governments monitor the conversations and whereabouts of their people.
The most explosive revelation is that in a few countries, authorities require direct access to an operator's network - bypassing legal niceties like warrants. It did not name the countries.
Vodafone has more than 400 million customers in regions stretching from the UK to Africa, India and Australia. The company said it was publishing the information as its contribution to the debate on government surveillance systems.
It said wires had been connected directly to its network and those of other telecoms groups, allowing agencies to listen to or record live conversations and, in certain cases, track the whereabouts of a customer.
"For governments to access phone calls at the flick of a switch is unprecedented and terrifying," the director of British civil liberties advocate Liberty, Shami Chakrabarti, said.
In about six of the countries in which Vodafone operates, the law either obliges telecoms operators to install direct access pipes or allows governments to do so.
Industry sources say that in some cases, the direct-access wire, or pipe, is essentially equipment in a locked room in a network's central data centre or in one of its local exchanges.
The staff working in that room can be employed by the telecoms firm but have state security clearance and are usually unable to discuss any aspect of their work with fellow workers.
Vodafone says it requires all staff to follow its code of conduct, but secrecy means that it cannot always verify that they do so.
Government agencies can also intercept traffic on its way into a data centre, combing through conversations before routing them on to the operator.
Edward Snowden, the National Security Agency whistleblower, has joined Google, Reddit, Mozilla and other tech firms and privacy groups to call for a strengthening of privacy rights in a "Reset the net" campaign.
Additional reporting by Associated Press