There are lots of different two-way intercom devices, it depends on the exact features you need.
When you talk about ring or skybell or nest, in addition to The video they have two features which distinguish them from traditional intercom devices.
One) they are handsfree for the person coming to the door. They don’t have to push a button, they just talk.
Two) the resident can use their phone rather than having to use a separate intercom device.
Do you need those two features? Or are you just looking for a traditional audio intercom where the person outside pushes a button to talk, and the person inside is using a specific intercom device?
The following is a typical old fashion two way audio doorway intercom. But it requires that the person push a button and it only works between those two devices.