Common sense tells me that in a high stress situation, I should focus on my own self defense first.
I think it really all depends on the situation. Obviously, if your safety was immediately in danger, save the phone call until after it's over. If it were me, and I had time to call, I'd call first. Dial 911, put phone on speaker, throw it into the seat next to me. Now there's a recording of everything that happened if worse comes to worse, you can still talk to the dispatcher if need be, but you have both your hands free in the event you need them to defend yourself. Sounds like a win-win...but ONLY if there's an opportunity to make the call.