Vodafone, the world's second-biggest mobile phone company, said government agencies in a small number of countries in which it operates have direct access to its network, enabling them to listen in to calls.
Security agencies across the world, and in particular in the United States and Britain, have faced greater scrutiny since Edward Snowden, a former contractor with the US National Security Agency (NSA), disclosed the extent of their surveillance to newspapers.
Snowden's disclosures caused an international uproar, showing that US and British agencies' monitoring programmes took in ordinary people's telephone and electronic communications.
Vodafone on Friday published a "Disclosure Report" which said that while in many of the 29 countries in which it operates, government agencies need legal notices to tap into customers' communications, there are some countries where this is not the case.
Vodafone said it could not give a full picture of all the requests it gets, because it is unlawful in several countries to disclose this information.
"In a small number of countries the law dictates that specific agencies and authorities must have direct access to an operator's network, bypassing any form of operational control over lawful interception on the part of the operator," the company said.
Vodafone has not named the countries where this can happen, but in the document it calls on governments to amend legislation so eavesdropping can only take place on legal grounds.