This is what I’m currently working on.
I spread a sheet of drawing paper on my actual desk and drew up a diagram. The idea is to use an existing local network cable run to route a phone line into the server room, connect it there to a splitter and a dial-up modem, and then — after the modem — bring the line back to the office over the other pairs of the same cable and plug it into the phone.
The diagram is probably hard to read in the photo; I’m thinking of redrawing it in Dia and posting it. No time for that just yet.
For now I’ve wired up the socket — it’s half working (only one of the two connectors rings through).
