I do not know the technical details, but I have a dead phonenet adapter that I took apart to see if it could be fixed (assuming what you are assuming, I was merely looking for loose connections, etc.), and yes, I did discover that there is a tiny bit of circuitry in there, mounted on a small board. That is what had gone kaput, rather than some soldering or whatever.

This is not to say that straight wiring along the lines of a serial cable could not work (presumably a set of telephone wires stuck in the relevant serial port holes, as if it were a cable, might work, but it would be less than ideal, to say the least). What it suggests, however, is that phonenet was something more than a wiring bridge/ connector/ plug.

Phonenet adapters can be bought for very little, so why not just pay for some? I have none to spare, but somebody on this list surely does. Post a WTB in the relevant forum.