The phone number that appears in a social media post is the title of a popular 1980s song, "867-5309/ Jenny" by the power-pop band Tommy Tutone. This number has circulated as an inside joke for the past 40 years, and it has nothing to do with reporting maskless people.
The song "867-5309" was released November 19, 1981. In addition to the song becoming a mainstay on classic rock radio stations over the decades, it became a joke for people in various area codes to dial 867-5309 and ask to speak to "Jenny."
The social media post, carrying on the inside-joke legacy, consists entirely of the number from the song, along with "To report someone for not wearing a mask call ..."
The CDC's real recommendations regarding mask-wearing are posted on the agency's website.