White House staff used to "babysit" President Donald Trump's phone calls with foreign leaders because he was often unprepared for the conversations, a source familiar with how calls to foreign leaders were handled under former White House chief of staff John Kelly told CNN.

Kelly left the White House in December. The source said Kelly would often mute Trump's phone calls so people in the room could guide him because he was known to sometimes go off "on random tangents" with foreign leaders.

A Trump phone call is at the center of a whistleblower complaint filed last month, which has since triggered a formal impeachment inquiry.

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President Donald Trump was so frequently unprepared for conversations with world leaders that White House staff needed to sit in on his conversations to make sure Trump didn't say anything inappropriate, CNN reported.

A source familiar with how calls to foreign leaders were handled under former White House chief of staff John Kelly told CNN that several staff members would coach the president during his phone calls to avoid any diplomatic gaffes.

"Kelly always wanted a bunch of us to be there in the Oval (Office) ... to just babysit on these calls," the source said.

In the past, officials from the US National Security Council would brief the president before a call and then sit with the president in the Oval Office during a call, according to the BBC. Other officials in separate parts of the White House will also listen in on conversations and take notes, known as a "memorandum of telephone conversation," which would often be combined with automated computer transcriptions.

The White House began to limit the number of people allowed to listen in on Trump's phone calls after transcripts of his calls with the leaders of Mexico and Australia were leaked, according to The New York Times. According to CNN, the current policy typically allows four officials to listen in on calls with foreign leaders, including "the national security adviser, the National Security Council director and deputy director for that region."

Read more: Trump is facing the biggest firestorm of his presidency because his own staffers blew the whistle on him

A source told CNN that before Kelly left the White House in December, he would often hit "mute" on the call so people in the room could guide Trump, as he was known to sometimes go off "on random tangents" with foreign leaders.

"We were there to coach him in real-time because he was so impervious to coaching ahead of time," the source told CNN.

The White House did not immediately respond to a request for comment from Insider.

A Trump phone call is at the center of a whistleblower complaint filed last month, which has since triggered a formal impeachment inquiry.

An unnamed intelligence official lodged a complaint in August regarding a July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky, saying that they were deeply troubled by a phone call Trump had with a foreign leader, particularly "a promise" Trump made.

The intelligence community inspector general, Michael Atkinson, deemed it "credible" and of "urgent concern," and passed the complaint on to acting Director of National Intelligence Joseph Maguire.

The complaint, which was declassified last week, alleged that Trump pressured Ukraine to investigate former Vice President Joe Biden. The White House last week also released a memo about the call, which appeared to confirm that Trump pressed Zelensky to open an investigation into corruption allegations against former Vice President Joe Biden and his son Hunter.

US House Intelligence Committee Chairman Adam Schiff said on Sunday that Congress was determined to get access to Donald Trump's calls with Russian President Vladimir Putin and other world leaders.

"I think the paramount need here is to protect the national security of the United States and see whether in the conversations with other world leaders — and in particular with Putin — that the president was also undermining our security in a way that he thought would personally benefit his campaign," Schiff said on NBC's "Meet the Press."