How does the authentication of surprise guests, cabs, courier, delivery boys etc work? (Visitor Validation)

For every visitor at the gate without a pre-approved OTP, we initiate an authorisation process with the residents of the flat. The security guard at the main gate takes the necessary information of the visitor such as the name of the visitor, contact number, picture, company, vehicle details and the flat that he/she wishes to visit.
When a visitor at the gate (cab, delivery boy etc) tells your flat number, the system will reach out:
1. First to all members of the flat who have the app installed. You will hear an in-app "tring-tring" two times. Any of the members in the flat can approve/ deny. Let us say member 1 has approved/denied then other members will know who has taken the action

2. If there is no response to the in-app calls after 50 seconds, the system will dial a voice call. It will go to the number designated as the "e-intercom" number of the flat.

3. If there is no response to the voice call, the guard at the gate will fall back to the wired intercom and try to reach you. For people, who at the moment have not downloaded the app, the intercom call would continue.