(CNN) President Donald Trump was often so unprepared for calls with foreign leaders that he was coached by several staff members and advisers as the calls took place to avoid veering off track, a source familiar with the calls through White House chief of staff John Kelly's tenure, which ended in December 2018, told CNN.

It's unclear whether a version of the practice was still in place when Trump spoke to Ukrainian President Volodymyr Zelensky over the phone in July 2019, a conversation flagged in an intelligence community whistleblower complaint that is now the basis for an impeachment inquiry into Trump.

CNN previously reported that as leaks from the calls over the course of the administration angered Trump, top officials in the West Wing began to limit who could listen in on the conversations, hoping to tighten the circle of people aware of what the President was discussing with foreign leaders.

These days, only around four officials are typically listening to foreign leaders calls, including the national security adviser, the National Security Council director and deputy director for that region. The practice excludes unofficial calls Trump makes on his personal cellphone.

According to the source familiar with how foreign leader calls were conducted under Kelly's watch, Trump often had to be reeled in by White House staff during calls with foreign leaders, adding that Kelly would sometimes mute the call so staffers in the room could give the President guidance or advise him against saying something he shouldn't say.

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

During the first year months of Trump's presidency, foreign leaders were often caught off-guard by these phone calls and the President's style.

"He would go on random tangents about the Mueller investigation with foreign leaders ... it was unnecessary and unhelpful," the source said. "And sometimes he just wouldn't have his facts straight and he would really rattle some of the foreign leaders with whom he spoke."

The source added: "Eventually they figured it out and they adjusted, but those calls were nothing like what a normal call would be between presidents."

Earlier in the administration, if the call happened in the Oval Office seven or eight staff members and advisers would be gathered around the Resolute Desk or listening in. The coaching by those in the room came, in part, because there was very little likelihood the President had looked at "read-ahead material" associated with the calls.

"We were there to coach him in real time, because he was so impervious to coaching ahead of time," the source said, but conceded that "if the President was determined to say something, you couldn't really press mute."

"There was no stopping him. But when there was a lag because a long translation was going on, Kelly would mute the call so that staff in the room could give guidance," the source added.

The source said they never heard a call as "problematic" as the Ukraine call cited in the whistleblower complaint, but there were still "politically embarrassing" and "inappropriate" incidents on the calls that had "diplomatic consequences."

These incidents included Trump "talking to one foreign leader about another in a completely inappropriate way" and Trump just generally not knowing "how to be diplomatic."

"He was so direct and blunt about trade deficits. No one would ever talk like that ... Sometimes what he would say wasn't earth shattering, but it would be very different from the public messaging or positions," the source said.