After consulting support and trying to find solutions online, I finally was able to get the Nuki working on my intercom. I’m posting this in hopes it helps others facing the same problem.
The Setup: 4 wire analog apartment intercom with 3 buttons. Note, this is analog intercom with mechanical buttons, NOT a bus system.
Wires: Audio In, Audio Out, Audio Common, Tone In
Buttons: Talk, Listen, Door
Soldered to the door button on the intercom are the Audio In and Audio Out wires. Pressing the door button opens the front door of my building. It appears that pressing the button shorts the Audio In and Audio Out wires, sending a signal to the door.
Wiring: I used levered wired connectors to connect the Nuki between the incoming wires and the intercom unit.
Nuki Blue- attaches to the Audio In wire
Nuki Purple- attaches to the Audio Out wire
Nuki Black- attaches to the Audio Common wire
Extra, to use Ring to Open
Nuki Yellow- Tone In (incoming wire)
Nuki Green- Tone In (intercom unit)
Pressing the ‘open’ button in the Nuki App apparently sends a signal through the blue and purple wires, shorting the Audio In and Audio Out wires. This setup works pretty well although it did introduce some low level, but tolerable noise through my speaker. I think I need to either secure my connections or possibly remove the Yellow wire from the Nuki. I hope others can use this information.