So let me get this straight. I have been kidnapped in my home by a lunatic who threatens to kill me if I try to call the police or escape. I manage to call 911 for help, and your suggestion is that the police call me back or ask permission to enter so that the kidnapper can make good on their threat to kill me.
1st That call wouldn't be anonymous.
2nd While I heard of hostage/siege situations on the news, in none of them were hostages shot at the first sight of a police officer. I know this is more anecdotal and secondhand, but even from the viewpoint of an armed madman, killing your negotiation material first thing is a very bad tactic.
it corrodes the (already strained) bond of trust between the people that need the police to protect them, the people who have to respond, and the people on the other end of that police response.
Yes. But if you have to be afraid of a swat team raiding your house and killing your 6 year old daughter because they got the wrong address (Detroit IIRC) or anonymous callers, THAT won't help rebuild that lost trust.