I guess I agree with the poster who suggested that the dispatcher should ensured the occupants knew the police had arrived. If the dispatcher did attempt to do that, this might just be one those unfortunate situations where no one was at fault--except, for the bad guy(s) who shot up the door to begin with.
Not only that, but they have been known to wear shirts/jackets with “Police” or “DEA” or “FBI” on them, and body armor as well. I have seen several cases where that ploy was used. I particularly remember a couple cases in Phoenix and one in Texas.