I have seen something like that. It essentially simulates a telephone
network. You would use it to test modems, for example, or to connect
machines that had internal modems. I think I saw it being sold by Black Box.
They are not the cheapest outfit in the world. It's like buying at the local
convenience store. IIRC they were over five hundred dollars.
I am surprised, but I have never seen a published schematic for such a
unit, neither in a magaxine or on the web. They can't be _that_
complicated, surely?
-tony