What I hope will be the final piece of the puzzle arrived today. It’s a little adapter that turns a regular modem into an acoustically-coupled modem. It’s intended for use when there isn’t a regular phone line available—you can strap this to a regular handset instead.
I decided to do a proof on concept, just to see if the modem+coupler could talk to the TDD. Presto:
It seems to work fine, and now I just need to embed that coupler inside a phone handset….