That is, the content of the talks is known only by the user who make and receive calls. Facebook cannot know the contents of the talks without turning off encryption. Facebook has argued if they approve the request of the U.S. Government, then they have to rewrite the code in the application to turn off the encryption that is relied upon by more than one billion people each month.
Facebook is not the first technology company requested the US Government to access data belonging to criminal suspects. In the year 2016, Apple engaged in Legal Affairs with the FBI after refusing to grant access to the iPhone's proprietary mass shooter in San Bernardino, United States.