I first got my start in this business using POTS lines to a cheap ATA 4xFXO card. In fact, my story mirrors @Ducktour’s almost word for word, except I threw out my Cisco’s and replaced them with Aastra (Sangoma phones wouldn’t exist for another 8+ years).
After A LOT of tuning with the el cheapo ATA card, I got echo down to a nearly perfect and used it in production for years. Except once or twice a week there would be a complaint about talkoff, or missing caller ID, or mild echo, or something else, so the system was working well but not perfectly. Despite easily hundreds of hours of messing around with fax, I never did get fax to work reliably enough for production, maybe 90% success (good but not good enough).
At some point I popped a Sangoma TDM card w/EC in the PBX and ALL of my POTS complaints immediately went away. This was years before I would ever come to work for Sangoma. So if your time is worth nothing, and you are keen to learn the system, then I can’t imagine there is a more thorough way of learning the internals then trying to make a poorly performing ATA card work to production standards. It is also the most frustrating, and not something I would ever recommend someone go through when there are so many other things to learn that are more satisfying.