Hardware first. Use an optocoupler: bridge rectifier (or a PC817 with a reverse diode across the LED), 1 kΩ series resistor, 10 µF across the output so the AC ripple doesn’t show up as 50 pulses a second. Transistor side to D5 with INPUT_PULLUP, so the pin goes LOW while it rings. Debounce in code, send ring: 1 on the falling edge, and ring: 0 when it’s quiet again for 2 s.
Then I’d skip the alarm rule. An alarm has a lifecycle (open, acknowledge, clear) and you’d have alarms piling up for every courier. A Rules card rule fits better:
When: A value changes, device "intercom-d1", value "ring", becomes 1
Then: Tell people [x] Members
Message: "Someone is at the street door"
Members get it in the bell, by push where they’ve set it up (the app, or a browser with push on), and in every chat app they linked under My Profile > Chat apps. So once your admin connects the platform each person links their own account and that’s your phone ping, app or not. Rules card recipients never get e-mail or SMS, for SMS you’d need a flow with a Notify block and phone numbers.
One thing to know: the same notice to the same person is held back for a 10-minute cooldown. Fine for a doorbell mostly, but if two couriers come within 10 minutes the second one only shows up as “(+1)” on the next one. Details: Notifications from rules and flows.