There is an old (1996) intercom system in our building. When someone presses my apartment number button at the gate, I get a "call" on a phone-like device at home and press a button to unlock the door strike downstairs.

Below is a link to the user-guide of the device:

I need to modify (rig/hack) it in such a way, that "open" button is "pressed" automatically 3 seconds after incoming call. So that there is no need to have person pickup the call and push the button.

The reason for that is that on certain days I need to let some people into the building without being present there. So ideally I'd like this rig to only work during certain windows (which I program manually) and not 24/7. I'll provide more detail on how it should operate.