[fast*st]

So if I recall reading things correctly, regarding the ISDN port on the Indy, it was to plug an ISDN phone into and place calls over the network.  Did anyone ever try/use that functionality?  It would of course require peeling off the big angry sticker.

[radiance]

Yeah,

Back in the 1990's, you could have an ISDN line installed, which was a special "digital" phoneline.
They would install an NT in your house, which is a box on your wall that has 2 ISDN connections on it.

You could plug one or two ISDN devices into it (like an ISDN phone) and make calls.
Because it was digital, there was no dial-tone, dialing noises and other handshake sounds etc... it connected much quicker.

One line was 64Kbits/sec and you could use use both lines together to get 128Kbits/sec for data calls. but offcourse pay double!  
You would call into an ISDN line at an ISP, very much like with an analog modem.

ISDN was not very popular, as it was expensive, still required dialing in and paying for the call, eg per minute,
and the difference in speed was abysmal, eg most modems were 28.8 <> 36 Kbits vs the 64Kbits of an ISDN line.

It was available for only a few years, and was quickly pushed aside by the much faster and "always connected" functionality of ADSL over analog phone lines...

I used to visit an internet cafe in Belgium sometimes which had an Indy (challenge S variant) which acted as the network's router to connect all the cafe's desktops to the internet via it's ISDN modem. very slow and very expensive

Radiance

[jan-jaap]

I think you could also make a voice and data calls at the same time and/or have up to four numbers on a single line. No more complaints about hogging the telephone line all evening with the modem.

My current house had ISDN cables going to various rooms when we moved in, probably because ADSL was troublesome in that area. I replaced all of that with gigabit ethernet, and FTTH for the uplink.

ISDN is still used by (business) alarm systems. They want to kill ISDN but keep postponing the end date.

[cybercow]

I would add few things, ISDN was full digital technology, whereas old modems were analog in nature, you could "hear" the transmission if picking up the phone while operating the modem. Second, although the ISDN 64 kbit/s vs eg. fastest modem at the time like 28.8 or 33.6 seems not much of an improvement on a paper, in fact it was really a NOTABLE BIG increase in speed because; modems were always suffering from line interference, and the max. speed they achieved was never near or close to the max speed stated by the manufacturers on those shiny boxes. On your best day if you had luck you were surfing at a fraction of that speed, inherently, incurably slow as dogs. ISDN offered you those "real" 64k out of the box, plus minus few percent, ISDN was SYNCHRONOUS uplink / downlink and even more - you could open 2 channels and get up to "real" 128Kbit/s which was really an impressive speed back then! So thinking of only double of triple time improvement over traditional analog modems is incorrect either in real measurable speed or in the reliability and overall responsiveness of ISDN. All in all ISDN was really a big leap forward in digital communication.