We don't know, but the CNN article author does, and that author says SPECIFICALLY in their article that the whistleblower had no direct knowledge of the phone call, and was basing their statement on something they heard from someone else - not in the course of their work (outside of work).
Otherwise known as "hearsay"
I don't care if you believe me, but you might want to believe the author of the article that set your panties on fire.
https://www.cnn.com/2019/09/20/politics/donald-trump-whistleblower/index.html