This is true. One time my ex-wife before she was my wife had a minor hissy-fit and curled up in a ball and began hyperventilating. I called 911. I didn't call back. During the call itself she got on the phone and told them she was fine (except the drama kind of backfired) and not to send anyone.
They dispatched anyway and I had two cops and a couple of EMTs drop by to run her vitals and determine that everything was indeed alright.
I may be wrong here, but I'm under the impression that once help is called for, especially if it has so much of a wiff of a domestic issue, someone is coming to your place regardless. As it should be.
Could you imagine the fallout should some woman be beaten, manages to call 911, after which her abuser beats her into calling back to cancel and they go right ahead and not bother to show up?
All it would have taken is for that to happen once for heads to roll and policy be changed.
P.S. What I learned in my 20s: Should your GF get involved in a tickle fight with her friends and fall head-first into an upright cast iron radiator and split her head and bleed all over the place and you, as a dutiful and scared s-less boyfriend drive her to the emergency room, be prepared to answer some questions. Lots and lots of questions. Even after they take her in the back of the ER before they even stitch her up and she tells them what happened and your total lack of involvement.
Again, as it should be.