At work we use iwatsu phones. At one time we had a device that could convert any analog handsets into a remote ip phone, but we can't remember what they were called. I don't think they were ATAs, because they could work with any system's analog phones and they could support all of the handsets features related to the system. But I could be wrong.

The only product I can think of that is even close to what you are describing are MCK Extenders, and we have been selling Iwatsu for almost 40 years.

MCK units worked utilizing a POTS line between the main location and remote location, as the connection medium. This technology is at least 10+ years old, and I think you could actually have an Iwatsu Digital Phone on the remote end. We only installed a few of these, as the IPSUB card was released shortly after.