Hi StooJ and welcome to VoIP User.
There's no such thing as a daft question, just daft answers (or lack of any answer

).
Let me make sure I understand this correctly. You want a basic voice/intercom system running on a LAN with no internet connection, with voice calls being initiated not by users, but by a central server?
What mechanism would be used to initiate the voice calls? Can the server make an HTTP post or similar?
If I'm correct in the above assumptions, Asterisk could be configured to do this (with an HTTP post callback coming from the central server).
You would need to have softphones logged in to each machine though, ready to receive calls (Linphone would be fine).