In fact, it's downright impossible sometimes.
People screaming, whispering, running, not talking into the phone, talking too close to the phone, on speakerphone, air conditioning blasting into the phone, and a million other things.
So misunderstanding is a very real hazard of the job.
Co-worker: "An officer is shot?!? Where?!?"
Caller: "A sir is shot!"
CW: "Wait, a sir - a man? Not an officer? No police officer shot?"
C: "A sir! A sir is shot!"
CW: "Who says that?!"
Co-worker: "You are reporting sick sale prices? That's what you are calling 911 for?"
Caller: "No! A sickle-cell crisis! Like bad blood. Like my blood is trying to kill me."
CW: "Ooooh, sorry. Let me get you to the paramedics... hold the line please."
While taking a call for a fender-bender...
DispatcherDame: "Wait, you wold me that you weren't injured... Why are you saying now that you've been shot? Have you been shot?"
Caller: [Yells what sounds like "SHOT!"]
DD: "Okay, you were shot- by whom?"
C: "No, not bang-bang, like boo!"
DD: "Oh, SHOCKED! You are shocked. Okay then."