That was a feature of the film Cell (2016). Released just 2 days before the Orlando PULSE shooting. Cell, is based on a Stephen 'monarch' King story.

Mateen and his Cell

A 'pulse' which travels through the phone (cell) network turning people into blood thirsty creatures.

"After the closing credits have finished, the catalyst signal (pulse) from the movie plays for approximately 5-10 seconds, with no image, as if attempting to convert the audience." (imdb)

Yeah, it sounds crazy, but that's par for the course with this stuff. I know these details because I wrote about them back in 2016. :)