How to set up a sip notification system using Asterisk + raspberry?

Question

Asterisk

Tell me, please, how to organize the idea, where to dig?
Task planned for solution:
There are 4 classrooms. 1 director, 2 warehouse, 3 accounting, 4 managers.
in cabinets 2, 3, 4 we install raspberry + speakers. We connect each raspberry to the local network.
We install Asterisk at the director's office 1, located on the same local network.
Install raspbx or raspbian + baresip on raspberry. The bottom line is to make a sip device out of it, which will automatically receive all incoming and output sound to the speakers.
It remains to figure out what means and how to configure the director's ability to make a call to the necessary offices (raspberry).
For example, in pom. 2,3,4 we play some kind of playlist or audio file.
Then in pom 2.3 the music continues to play, and in pom. 4 turn off and say something into the microphone.
Then we decide that we want to speak into the microphone now in pom. 2, 4, and at 3 turn everything off.
In other words, with the help of sip and raspberry, I want to make local ip columns.
As I understand it,
1) on the rspberry side, you need to set up an auto-reply for all incoming messages, incl. if if someone is talking.
2) On the Asterisk side, you need to set up conferences for each case
-- 023 for help. 2.3
-- 024 for pom. 2.4
-- 034 for pom. 3.4
3) when pressed through softphone_1 * 023 in pom. 2 and 3, an outgoing call is created, which the rasberry automatically accepts. And everything that the director now says into the microphone will be heard in the speakers until the director ends the call.
3.1) softphone_2 works in the background, the sound output from the media player is set to the microphone input as a virtual cable. According to the method from paragraph 3, he also makes a call.
Now most of all the question is how to implement point 3 (apparently the answer lies in point 2). Because all I found on habré is that each rasberry should ask itself to join the group (conference)
Or I complicate everything and "everything was invented before us" and there was already some kind of article that I could not find. Or, in fact, for the purposes of creating a broadcast on ip columns, as well as creating them, there is another solution.
Of course, I know that there are IP columns, 2N or CyberData, but it's too expensive. Especially when there are an abundance of ordinary speakers and raspberry are gathering dust.

Answer 1

seems to have found a solution, I will test
But the question is relevant. Maybe someone faced similar tasks and can share their experience. Maybe there are easier solutions.
Especially in terms of management. Client.
It is difficult to store and create many different options - groups, especially when scaling.
It's easier, like in a smart home - I chose the checkboxes of the devices to broadcast to and chose what to broadcast (file, input source (microphone)) and pressed play. The system automatically created a conference and called everyone to automatically add them