I have some old modems (TRS-80, acoustic and etc.)
which I would like to
use (flashing LEDs are cool) so, I want to build a little telco emulator to
interface with the modems in one of my Linux boxes.
Circuit Cellar ran a series of articles on doing this sort of stuff; they
outlined the design of a simple mini-PBX, and gave sample schematics of
portions of it.