That was an inspired commercial.
For purposes of the OP, this happened within my experience. Or “happened” if you will.
In the 1970s a UFO was reported to have landed in our area. I lived in the area but was not on the force at the time. But I did join 3-4 years later and I had the opportunity to grill one of the deputies that responded to the calls and was involved in the incident.
In short, the reporting went to Sheriff’s office dispatch to, which seemed logical at the time, a local Air Force Base, which apparently reported it up the chain. Hilarity ensued, but understand that everyone involved knew something “not right” and very scary was happening and no one had an explanation. Aliens/UFO seemed as likely as anything else.
I’ve been meaning to write a full recollection of my interview; maybe this is my incentive.
It will seem a severe overreaction, but you really should hear the story as it unfolded, because then the fears, and the conclusions arising therefrom, would be more understandable.
It was near 40 years ago when I interviewed my co-worker/Deputy who was there, and he being a Southern Gentleman, there’s a lot of twang and emphasis and nuance to his story, which gets lost in text. These were seasoned veterans, and they were scared shitless.
It obviously wasn’t aliens, of course. Give me a day and let me write it up proper. I owe it to John-boy and J. Paul and Starch.
As an alternative, I would probably report it as a mass shooting with at least 4 shooters with automatic weapons and body armor, along with as realistic of a casualty/wounded count as I could estimate.
This would ensure the maximum possible armed response turnout. When they arrive, they can worry about accurately reporting the actual threat.
I have written and posted the whole story in MPSIMS:
I recall becoming increasingly concerned bordering on real fear / alarm the one and only time I saw a UFO. I’m a professional pilot; I know what is, and isn’t, to be seen in the sky. This did not belong there. I was astonished at how unsettling it was as I struggled to comprehend what I was seeing through the dark, cold, drizzly, and foggy conditions. Oh shit, what is it? No, it can’t be. Can it? What if it is? Arrgh!
It was a streetlight in the suburban parking lot I was standing in, having just walked out of an eatery. The same eatery I visited roughly weekly for several years. But for 15 or 20 seconds it really had me going.
Hysteria can be a very real factor. Had what I saw gotten weirder or had more people been involved it could have snowballed to be a much bigger deal.
I’ve seen a couple of UFOs in my day.
The first was a bright flash of light in the pre-dusk sky, just before sunset. After looking for a moment, discovered it was an airplane with sunlight bouncing off at just the right angle to pop out at me.
Second UFO was a shiny orb moving across the sky. Figured out it was a mylar balloon from a nearby apartment complex publicity.
Neither case did I freak out because I knew there was an explanation, and it came quickly.
There are already multiple companies providing livestreaming video access to call centers. One of them, Prepared 911, had about 400 dispatch centers in the US using their tech in 2022 so the number now is probably even more.
I don’t remember how I acquired it, but I have a t-shirt from that ad campaign with the dog saying “Here, lizard, lizard, lizard!” Appropriately, I currently wear it when playing the Attack of the Giant Monsters: Atomic Edition roleplaying game. When we encounter giant monsters in that game, we never bother trying to call the authorities. Our mad scientist breaks out his ray gun, our juvenile delinquent flies around with his jet pack lobbing knockout pellets, and my mystic-powered luchador grows to 50 feet tall and grapples with the beasties. 
If I lived within a rural/forested area and some kind of “monster”…Bigfoot/Werewolf/Wendigo…is causing a mass casualty/fatality event, I would try to raise somebody on the horn…somebody with SERIOUS firepower…and say we have an emergency with a Grisly Bear with Rabies…NO, not cocaine, RABIES…and we need to put this thing down NOW!
On a dark country road I was being followed by a UFO. Until I stopped and realized it was Venus.
I’m not sure about the US military, but in the Canadian military the phrase “No duff. No duff.” is used to indicate that the next message is real and serious. It is usually used to report a real injury when on exercise. Nobody uses the phrase “no duff” without meaning it because it really is a life or death thing. It is heavily reinforced that using that phrase without justification is a severe career limiting move.
Now, that being said, if I got on the radio and said “No duff. No duff. A giant monster is attacking downtown Toronto.” I don’t know what would happen, but it is certainly what I would try. If it was not believed, then I would explain it more simply “This is no joke. Send me to Club Ed (military prison) if I’m lying. There is a giant monster attacking downtown Toronto.”
That’s when the heroes somehow manage to dispatch the monster and cover its tracks before the army appears, and you end up in a loony bin until the sequel, where the monsters attack again and you are vindicated.
LMAO! That’s fantastic! 
We need to have a new “Finish the…” story, like were written some years back. Make it a giant monster story. We know all the traditional plot points.