GXV-3500 Alarm Input not activating dial number



I have a new GXV-3500 here on my desk, latest firmware Everything seems to be working, including Alarm Out, while for the life of me I cannot seem to be able to make Alarm IN work.
My goal is to connect it to an intercom button: visitor pushes intercom button, GrandStream snaps picture of his/her. I am planning to use a double-button so as to apply a clean 12V DC voltage to the Alarm IN terminals.
I studied the wiring scheme at page 24 of the manual. I understand that Alarm IN is a polarized voltage sensing port: apply a 2.5V to 45V continuous DC voltage, according to the polarity or the ports, and that should trigger the alarm. It does NOT.

I tried the following:

I went to Advanced Settings - Alarm Event, and added various sensing schedule including “Everyday from 00:00 to 23:59”. I made sure the settings were memorized.
I went to Basic Settings - System - DI and DO. Tried both with “Normally Open” and “Normally Closed”. No matter what voltage I apply to Alarm IN, it always says “Current State is Open”
In the Home windows (live view), the little bell icon above the video image never turns red or changes, no matter what voltage I apply to Alarm IN terminal.
Instead of applying a voltage, I simply try to connect the Alarm IN - and + terminal with each other, nothing happens.
with a tester, I measured voltage and resistance of the terminals, both with “Normally Open” and “Normally Closed” settings, and shows 0 voltage, infinite resistance.

Am I missing some settings? Is my GXV-3500 defective? I am 100% sure I could not fry it with my actions, I have no access to more than 12V and I applied it with the correct polarity.
Can anybody who made Alarm IN work chime in?