As someone working with both VoIP and Fax, all I can say is that both are horrible, and expect the worst from each when putting those 2 things together.

Since my work is a large org (they still use fax officially) I mostly work with enterprise software from Cisco, Oracle, etc. so I cannot help here in detail, start learning about SIP (abd RTP if you want to handle calls too) with FreePBX. There is a Fax plugin for FreePBX: https://github.com/FreePBX/fax but I could not find any documentation so far.

The world would be great if we started developing new digital solutions to analog problems instead of forcing the digital replacement to mimic its analog original

• me

Yea, how much do you know about SIP?

I ask because what I would do in this situation is run up a PBX, like FusionPBX, and get the ATA units to connect as an extension to allow calls between them.

No need for a sip trunk to a phone provider as you don't care for external calls.

My recommendation is to start with getting fax to work locally. As in, from port 1 of a single SPA2102 to port 2 of the same. This would validate that your fax machines and the SPA2102 is operational, and is just entertaining in its own right to have a dialtone that "calls" the other port.

Fortunately, Gravis from the Cathode Ray Dude YouTube channel has a writeup to do exactly that, and I've personally followed these steps on an SPA122 with success, although I was doing a silly phone project, not fax project. https://gekk.info/articles/ata-config.html

If you're lucky, perhaps fax will Just Work because your machines are very permissive with the signals they receive and can negotiate. If not, you might have to adjust the "fax optimizations" discussed here: https://gekk.info/articles/ata-dialup.html

And then once local faxing works, you can then try connecting two VoIP devices together over the network. This can be as simple as direct SIP using IP and port number, or can involve setting up a PBX that both devices register against.