Hi,
I think that going back to the guides will be a good way to figure out how everything works together, even as you await some sort of advice from the more experienced members of this community. It may be a bit of 'trial and error' but i believe it will be a good way for you to better understand the technology. I am enclosing some links about the topic, which you may or may not have visited.
I find this fascinating myself, and would love to play around with this technology if I had the time and resources. I got into networking just as circuit switching technologies such as these were being aggressively phased out, and so I ended up not really getting deep into them.
Asynchronous Connections
Modem-Router Connection Guide
Configuring Dialin with the NM-8AM or NM-16AM Analog Modem Module
Configuring Dialout with the NM-8AM or NM-16AM Analog Modem Module
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