First of all, I don't think it was a 911 call. It was a call to the SPD dispatch. He was talking directly to the police. Second, there is nothing in the phone call that indicates Zimmerman confronted Martin. In fact, the opposite is true. During the call Zimmerman tells the police dispatch that he doesn't know where Martin is.
Zimmerman told dispatch that the kid was running and he wasn’t going to let the kid run. The only reasonable conclusion I can draw is that Martin wasn’t looking for a confrontation, but Zimmerman was, probably to apprehend the kid for the cops. The fact that the girlfriend is able to testify that Martin asked Zimmerman why he was following him strongly suggests Zimmerman was not in retreat mode by any means and Zimmerman had managed to locate Martin.
But he sure found him, didn't he? Zimmerman left his vehicle on the road and followed him on foot down the walkway.