Marina Gross, a translator and long-time employee at the US State Department, was the only American to listen in on the world leaders' conversations.
According to Politico, Ms Gross told her associates that listening in on the men was like "eavesdropping on two friends chatting in a bar".
It is unclear if that statement was made in response to the tone of the conversation, the familiarity between the men, or the subject matter that was discussed during the calls.
The two men spoke over the phone at least a dozen times during Mr Trump's presidency.
The content of those calls, while secret, will be accessible by Joe Biden.
“They don’t need our approval to see those [records],” a former Trump White House official said, referring to Mr Biden and his national security team. “Biden owns all the call materials. There is only one president at a time.”
Mr Trump reportedly did his best to prevent his conversations with foreign leaders from leaking, going so far as to hide the memorandums detailing the discussions behind the National Security Council's top-secret codeword system and stealing his interpreters' notes of his conversations.
Another former Trump official tried to argue that the former president's conversations with Mr Putin should remain privileged.
“There are certain things a president and his immediate staff should be able to hold privileged to do the work of government, without being subject to constant partisan gamesmanship,” they said.
Kel McClanahan, the executive director of the National Security Counselors law firm, told Politico that such privilege would be a pipe dream under any interpretation of the law.
“The only person who can claim executive privilege anywhere is the sitting president,” he said. “So there is literally no situation, nor could there be, where a former president could keep a sitting president from seeing something.”
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