I haven't seen that device but I was thinking how easy it would be to build something like it. If I had a hearing impaired friend who needed one, the intercom interface would be the only unknown. Doorbell voltage is either there or it isn't. Telephone has a huge difference between DC Tip & Ring and AC Ringer. The intercom is the only thing that I didn't just design in my head. I would opto couple all three to drive super bright LEDs on the annunciator.
Joe
