AFAIK, you can connect modems without using the telephone lines. Google "modem to modem direct connection".

But maybe you want more than a computer<->computer connection, and more like a printer server. If it's a USB peripheral, then you'd really have to know embedded systems.

Otherwise, for an old-school RS-232 peripheral, it might just work. Or controlling UCs (micro cont). Most can speak simple serial protocols.

Of course, a simple null-modem cable would work, too. ;-)

Current-loop serial interfaces are good over long distances as well, if that's what you need. Research the old teletype lines (and MIDI interfaces use current loop, too).

Yeah, and might not work as far as the next room.

Anyway, good luck.

.  You can do that with telephone modems, but a null modem cable works better. You can use a null modem cable, but Ethernet is faster. Most computers have E'net built-in, but, if you need a card, they can be found for < $20.

.  line simulator (modem to modem)
.  null modem (serial port to serial port)

.  Ethernet is pretty much the RS-232 of today (right behind USB). If you could install a serial printer back in The Good Old Days, you should be able to handle TCP/IP.
.  Much like you don't have to be able to rebuild a motor in order to drive a car, you don't have to know a lot about E'net/TCP/IP to get a small network to work - if you have setup a router at home, you probably already know just about everything you need to know.

If they want to send and receive faxes from their computer rather than using a service. I use to have DSL and I would have my phone line go to a splitter one side going into my DSL Modem and the other side going into a filter going into my modem for faxes. Worked great.

magic... i use arczip... google can sometimes take up to 10 seconds to load. the instructables home takes ~ 2 min 20 sec to load with all the pictures.