I just didn't expect all that for such simple equipment like ISDN modem. Just see for yourself it the pictures below and remember that this is late 90s and it's just a modem.
Well, it's not "just a modem" at all! ISDN is actually quite complex and required a good bit of technology to fully implement.
In addition to acting as a high speed digital modem it also provided two separate voice channels. One of those phone line connectors on the back of the board would provide two phone line outputs (the other was the ISDN input). You see, it was the modem's job to to locally create two analog phone lines; it would then digitize each line and use one of the 64Kbps "B" channels to send the G.711 encoded voice data to and from the central office.
So, you essentially had two 64Kbps channels (called B or bearer channels) which could be used for direct digital data or digitally encoded voice data. So, let's say you were connected to an ISP over both B channels at 128Kbps; if you picked up the phone (attached to the back of the modem) to place a voice call, your data rate would drop from 128Kbps to 64Kbps, and automatically go back up when you finished the voice call. If you used both voice lines, your data would essentially stop (unless you used something like X.25, in which case you could still have a 16Kbps data connection over the D channel) and would resume when one of the voice lines was hung up.
The idea was, you could provide high speed data access (and 128Kbps was high speed for the time) and two voice channels, all over a single copper pair. (Which would traditionally require 3 or 4 separate pair for the same level of service!)
This was perfect for small offices and stores. In some European locales, it caught on for residential use as well.
The unit you took apart appears to be a design from the early 90's, as by the late 90's, most new ISDN modems were coming with Ethernet interfaces (acting as routers). (That's not to say yours wasn't made in the late 90's; it could very well have been!)
I really miss ISDN... I remember getting it in early 2000 to replace my dedicated 56k dialup connection. I would have gotten it sooner as the service was super cheap ($20/mo to the Telco, just for the line), however there was a catch: The telco charged *per minute* (on *each* B channel) for outgoing calls.
What changed is I made friends with--and started interning for--the owner of a local ISP. You see, I discovered a loophole: While outgoing calls were toll, incoming calls were free! So, I setup an "ISDN Dialback" connection in his system. Basically, my ISDN modem would call his primary dialup, authenticate and then immediately hang up (this happened in under a second, so I wasn't charged). His system would then immediately dial my modem back, which would answer, authenticate and then bond both B channels. This essentially gave me permanent 128Kbps (up and down) internet service, and it was freaking awesome.
I remember the first night we got it working, joining a Quake III match and absolutely dominating. It helps when your ping goes from 500ms to <10ms! (I ended up starting my web hosting business on that ISDN line later that year, which grew into a very successful company!)
Anyway, ISDN was pretty cool. The reason your board appears so complex is because it *is* complex! I'd wager that at least 75% of that complexity is in providing the two analog voice lines. If all it had to do was convert an RS-232 serial interface into ISDN signaling, I suspect it would be a lot less complex. But "emulating" two POTS lines means generating a ring voltage, digitizing the analog voice channels, etc. which can take up a lot of space!
Welcome to the forum, and thanks for the blast from the past!