In your 'data center' you have you BBS happily running in a cramped little corner, with bunches of wires and whirring fans, but little room to sit at the console. Elsewhere in the room you have one or more other computers that you either connect to the BBS locally (modem to modem) or to the outside world. Reconfiguring your local wires to connect to the BBS or outside usually means plugging and unpluging wires between various modems and the wall outlet, which can be a mess and a pain to keep it all right in your head.
This project will present a more elegant solution.
Basically what the switch does is let each computer in the chain of phone wire, access either line 1 (the inside red/green pair of a phone wire) or line 2 (the outside yellow/black pair of a phone wire) and also disconnect line 1 from the wall (while still preserving line 2 for any outside computer access) so the computers in the chain can connect together locally or outside.
The parts for this project are relatively simple to locate and building it is small task with a bit of soldering and some drilling/filing to make a nice project box for the circuit.
A visit to a 'dollar store' (for the phone wire and doubler and maybe the box) and a Radio Shack (for the switches) should get you all the parts for under $7 a unit.
Here is a picture of a completed unit (I found an outlet tripler, the middle jack is to the modem, one on the right is toward the wall and the one on the left toward the next modem/switch box down the line. I arranged the switches so it made sese when you plugged in the wires (the → symbol next to the SPDT switch is ON, or connect line 1 toward wall, the numbers 1 and 2 indicate selecting lines 1 and 2 respectively.)
Here is a picture of the underside which gives a good view of the wiring I made, (remember this is flipped over so think the other way from what you saw above)
Example of how it all works (this is a three computer environment; a Commodore BBS, a Mac and a Commodore 64 - who could want anything more? Any combination of computer will work, just needs a modem with a modular wire connector):
Here is the 'normal' setup for a BBS sysop, the BBS is connected to 'the BBS line' (line 2) to get incoming calls from users, and line 1 is available for the other two computers in the room to access (of course not at the same time, that would be nearly impossible.):
The Mac's box was changed to line 2 and the BBS to line 1, and the line 1 access to the wall was disabled at the BBS too. Cool, eh? You can also link the Mac and 64 together or the Mac and BBS and still have line 2 access with the other machine…
Besides the wiring you may need to do some configuring of your terminal and/or modem to make a successful null-modem connection.
If you have never done this before you might want to test this out before building this project by just stinging the modems together with a wire (both connect to the line port) and testing/practising especially to determine if the BBS has a 'force answer' feature.