Let's get this right: it was not 911; it was the Non-Emergency Number (NEN) operator/dispatcher.
He had already instructed Zimmerman to "let us know what he's doing," which would imply a direction to maintain visual contact.
Martin walked PAST Zimmerman's parked car and stared Zimmerman down. THEN, Martin ran between two buildings, so that Zimmerman, still seated in his car, could no longer see him.
Zimmerman exited his car to try and see where Martin went.
It was THEN that the NEN operator asked Zimmerman if he was following the suspicious person. Zimmerman replied, "yes."
The NEN operator then said, "we don't need you to do that," to which Zimmerman replied, "OK."
Zimmerman then looked for a house number to direct the responding LE officers, and it was then he claims Martin confronted and attacked him.
Well done. Good summary!