Just an idea as I did something similar with an Elcom system:
Maybe you have a speaker or an LED that lights up in the intercom and you can meassure a voltage against the same “ground” as your opener signal already uses.
In this case you could try to solder the yellow wire here to get a ring notification.
This worked in the case of the Elcom system.
You can see some wiring diagrams and photos in this thread: