Procure an old smartphone, pop in a square LTE capable SIM or if possible arrange for WiFi.
Create a common email address for the entire apartment block that can be used for the administrator account and set up phone with it.
Download App lock and set it up with a lock pattern/password. Using this app you need to put the phone on complete lock down. Start with toggling on the “prevent installing and uninstalling” feature under play store so that no new apps can be installed or uninstalled. Next Toggle the “prevent WiFi toggle” lock. Scroll on below and lock contacts, settings and phone system apps as well. Then switch to the right tab and navigate to the general settings. Turn on advanced protection by downloading the required app for it. Locking down will ensure the phone will only be used for e-intercom duties and nothing else.
Download find my device and log into it with a google account you made in step 2 and make sure your device is ring able and locatable with another phone or laptop.
Store the phone numbers of the house owners as contacts with their house numbers as the names for simplicity.
Download Google duo. Click on the options icon on the top right and make sure the ‘Knock Knock’ feature is turned on. Now the residents can look at the video without actually having to pick up the call.
You have now a limited but effective e-intercom system that is way cheaper than any professional solution.