Chile's mission to the United Nations was spied on in the run-up to war in Iraq, the former envoy has alleged.
At the time, US officials were trying to make the case for invading Iraq
Technicians inspecting the telephones found they had been tampered with, Juan Gabriel Valdes said in an interview.
At the time, Chile was one of several countries seen as undecided on whether to back a proposed resolution sanctioning the use of force in Iraq.
UK media reports at the time alleged the US was monitoring communications from envoys from those countries.
Those reports kindled his suspicions, Mr Valdes said in his interview with Spanish newspaper El Pais.
"We called technicians to see if the Chilean mission's phones at the UN had been tampered with," he said.
"The result was positive. We discovered the great majority of phones had been tampered with."
Mr Valdes is the current ambassador to Argentina.
In the first few months of 2003, the United States, Britain and Spain were trying to ensure that at least nine of the 15 members of the UN Security Council would back a new UN resolution explicitly sanctioning armed intervention in Iraq.
France, China and Russia had already signalled their opposition, and supported continued weapons inspections in Iraq.
Six countries - Chile, Angola, Cameroon, Guinea, Mexico and Pakistan - were thought to be undecided.
Press reports in the UK suggested the US National Security Agency was spying on the envoys of those countries, and had requested British help.
According to El Pais, the aim of the alleged surveillance was "to facilitate an advantage towards obtaining of a result favourable to those responsible for the US line".
The Chilean interior minister told La Tercera news website he would not comment on the allegations, saying he had "no information".