3CX is a free (if self-hosted) PBX solution that runs e.g. on Ubuntu.
It allows for video conferences, chat between all users, obviously phone calls, etc.
It has apps for Web, Windows, Android and iOS.
I use it as personal phone system to route to the cheapest SIP provider and provide a landline number for use for the family.
no official docker exists, there are two who provide a docker for amd64 and another for arm and i tried the one for arm which does not seem to work even though it was specifically made for raspi4