Call handling during a lock down

We are implementing a lock down procedure at my work. To make it effective we need to accomplish several things easily:

  1. Have a single special feature/extension for staff to call; i.e. **0000
  2. Calls to that feature/extension must
    a) Turn off DND on all phones
    b) Barge in to calls in progress
    c) Put all phones on speaker
  3. The person placing the call will make an emergency announcement and hang up

We are able to use paging to speak over the speaker in normal day to day operations. I figured there must be a way to meet our requirements listed above. I’m just unsure where to start.

ChanSpy seemed like an option for the barge-in but needs a specific endpoint.

By making DND controlled by Asterisk, not the phone, we can turn it off en-masse. Right?

I’m looking for any helpful hints.