I am also having the same problem.
Because my Nutone intercom was not on either the “works” or “doesn’t work” list, I did not even try to connect it directly. Instead, I isolated the two systems. The Ring, powered by a 24v transformer, simply drives a relay (24v coil, 57ma hold-in current). The “Pro Power Kit” is connected across the relay coil, as if that were the solenoid on a doorbell. Contacts of the relay emulate a button press for the Nutone system, which uses a separate, 16v transformer.
On startup, everything worked exactly one time. Resetting the Ring by holding down the side button for 30 seconds caused it to work exactly one more time. At first, I thought I had a bad unit, so I called in, and was sent a replacement (call it #2), and sent #1 back.
Unfortunately, #2 worked exactly the same as #1, so I called Ring Tech Support again. After having to talk for over an hour with a polite young woman on Ring’s tech problem line, finally got connected with the Advanced Tech Department, a person named Austin. After much discussion, Austin sent me another replacement unit (#3)–which does exactly the same thing. Called back, spent another hour with another polite young woman, who eventually tried to connect me with Austin, but finally said she would have him call me back. That was more than two days ago.
One of the tech service ladies I talked to had me short the two wires going to the Ring doorbell. That, of course, activates the relay, and causes our Nutone doorbell to ring normally.
Using an oscilloscope, we can watch the voltage across the Ring device. When the Ring button is pressed, it should momentarily emulate closed contacts, causing the voltage across the Ring device to go very low or zero, thus causing the relay to close, and thereby causing the Nutone to operate. When initially set up (or re-set up, which I have done many times) it works–once. By working, I mean the Nutone rings, and any connected Ring system devices also sound. But the next times the Ring button is pressed, the Nutone does not sound, even though other Ring devices do work normally, telling us that someone is at the door.
One person who commented suggested switching the Ring doorbell type to “None”, then switching it back to “Mechanical”. It worked for him, it did not work for us.
Since three separate devices have all worked the same way, this does not sound like a “bad unit” issue. Something is wrong with the software/hardware interface. It is time for Ring to admit the problem, and let us know when it will be fixed.