The appalling breach of security was discovered while the Tory leadership contest unfolded over the summer - but a decision was taken by PM Boris Johnson and Cabinet Secretary Simon Case to suppress it, security sources have told a newspaper.
It is believed a year's worth of messages were downloaded from the Foreign Secretary's phone, ranging from classified secrets to politically damaging internal disagreements, the Mail on Sunday reports.
The hack is thought to have been carried out by Russian agents.
It is also understood the conversations included discussions with foreign governments about the war in Ukraine, and criticisms of Mr Johnson made by Ms Truss to Kwasi Kwarteng, leading to potential blackmail.
Discussions with foreign ministers are believed to have disclosed precise details of arms shipment for Ukraine.
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