An email sent to or received by Clinton on this day has the subject heading: “Re: New cell.” It won’t be found in the over 30,000 Clinton emails given to the State Department in December 2014. Thus, the details are known because she will be asked about it in her July 2016 FBI interview.
According to a later FBI report, “Clinton stated she was familiar with the phone number ending in [redacted] referenced in the email. She believed the number was that of her BlackBerry because she did not recall using a flip phone during her time at State, only while in the Senate.”
However, in the FBI Clinton email investigation final report, evidence will be mentioned that Clinton actually had two phone numbers. One was for her BlackBerry, which she used just for emails, and one for her flip phone, which she used for phone calls. (Federal Bureau of Investigation, 9/2/2016)