WASHINGTON — Sometimes President Trump is in the Oval Office when he talks on the phone with a president or prime minister. Occasionally he is flying aboard Air Force One. Often he is in the White House residence, picking up the phone from the Treaty Room, or from his private suite, to connect with a world leader.

And at least on his end of the line, only a small circle of aides are privy to the conversation.

Ever since Democrats revealed that a whistle-blower in the intelligence community had filed an “urgent” report off a call the president held with a foreign leader, speculation has centered on how the person could have known what was said between Mr. Trump and Volodymir Zelensky, the young new leader of Ukraine.

With some difficulty, it turns out.

While Mr. Trump is on a call, his national security adviser is typically either in the room with him or listening in from his West Wing office. Typically, the adviser is joined by the senior director for the head of state’s region, as well as intelligence officials working from the White House Situation Room who connect the call and help to produce a rough transcript almost immediately after its conclusion.

The secretary of state can listen in if he requests, according to one former administration official. Vice President Mike Pence will also frequently join Mr. Trump on a call with a foreign leader, often at the request of Mr. Trump, according to an aide.