I have a SIP doorman and Gigaset SIP handsets, I want to connect all this little world locally via the SIP protocol.

I get FreePBX on Raspbian 10 Buster

http://download.raspberry-asterisk.org/raspbx-10-10-2020.zip

available at http://www.raspberry-asterisk.org/downloads/

I decompress the file to have a .img file

I burn an SD card (8Gb)

I'm taking the latest portable version of Etcher.

For example version 1.18.8

https://github.com/balena-io/etcher/

I burn img file to SD card via Etcher

I add an SSH file

To allow SSH, I add an SSH file to the boot disk and then eject the SD card

For the record the login is root/raspberry

I open FreePBX in the web browser

I blocked the raspberry IP on 192.168.1.103 on the router, so I use this IP.

We configure this first screen 

I access the administration 

I create extensions

In Applications / Extensions / Add Extensions / Add New SIP pjsip

I add a call group

When you ring the doorbell, you have to ring all the posts, so a call group.

My group will have the number 800.

To read: https://boxip.fr/doc/configure-ses-phones-ip-avec-freepbx/

I add a Manager User

I think about allowing the addresses of my network in Permit (or at least that of my Home Assistant). 192.168.1.0/255.255.255.0 allows all addresses from 192.168.1.0 to 192.168.1.255