Redial or dial...makes no difference. When Trayvon saw him using it, that’s when he probably punched him. I believe it more likely that Zimmerman already had the phone out and in his hand, since he had been using it to communicate with the 911 folks for some time.
Time and the investigation will tell. It may be completely moot if they determine there is not enough evidence to go to a grand jury.
It does make a difference. If he is carrying in his hand he can unobttusively hit redial. When the operator comes on line George has an earwitness.
Instead Zimmerman has said that he "reached for my phone". That can look like a threatening action and when Martin saw that, that’s when he probably punched him.