Just because you intend to use the 911 hotline as an emergency resource surely shouldn't impede my right as a phone owner from calling it in the manner you intended. It's my god-given right as a phonebook owner to call whatever numbers I see fit in whatever quantities I want to.
It doesn't even have to be 911. You can DDOS the phone lines of any business to make them less effective. Is this ethically defensible?
This is more the equivalent of replying to every piece of physical junk mail that you get using their provided self-stamped return envelopes. I think you'd be hard to pressed to argue that, while being a jerk move, you're still aren't entitled to do it.
There were stories of people who would attach their return envelopes to heavy objects and try to send those back to them.
Maybe some day advertisers will come to understand that if their ads affect me negatively, then I will do my best to make it unprofitable for them to present them to me.