The Leaker is most likely McCain.
If McCain was in the room possibly, but this is a detailed transcript, so someone transcribed it..my guess..there is a bug in the phone
McCain certainly is among those who were aware of the contents of confidential telephone conversations between Trump and foreign leaders even before any transcripts were made public, he revealed as much when he was pranked by those Russian comedians pretending to be the Ukrainian Prime Minister, or whatever the prank was, something like that.