> This is really a very interesting idea. And it should be relatively easy
> to implement. Since all modems sold conform to various v42bis v32, v34
> and other standards...

Not so. Telebit has a proprietary protocol called PEP in their Worldblazers.

> Not exactly, but modems use strict standards, and there are at most 50
> frequences or less in use. Piece of cake :)

PEP divides the spectrum in 512 channels, and transmits 2-6 bits per baud
through each channel at 7 baud, for a maximum bit rate of 21504 bits per
second. Not so fast, but it's damn near unblockable and works over phone
lines that would be hard-pressed to manage 300 baud FSK dialup.