Let's change yesterday's scenario (MGTOW Communications). The cell phones are working, so you don't have to screw around with radios or try to make an amateur ambulance run.
911 dispatcher: "Nine one one, what is your emergency?"
You: "Medical emergency, possibly life threatening."
Stop there. You've set the scene. Let the dispatcher ask, and you answer with the minimum of words.
911 dispatcher: [Next question?]
You: [Brief answer.]
Here are two articles, first the short one, then the long one:
How To Call 911 Effectively