So what is the concrete proposal for intercom and its competency & value ?

SHORT ANSWER:
Traditional telephone system with an on-premise PBX and numbering plan that is derivable from flat no., Tower and block (and fixed for life) to minimise directory size and maintenance across 1600 communication points. Low cost, High Reliability, Ubiquity, and ease of use across all age groups, residents, helpers, security, maintenance staff etc are the key value offered by such a system. No need to invent the wheel and use what medium-to-large enterprises use.


DETAILED ANSWER:
By Simplicity and traditionally proven and reliable technology, rather than an emergency or fancy  or self-serving and self-declared revolutionary solution. Here is how:

First, we want to use traditional analog-phone and not IP phone/Cellular phone etc which is powered by household supply or battery but by the network itself. It has following advantage:

1. Simple to use across ages - Senior citizen, Kids, Helpers and even tech savvy residents/guests can use landline phones
2. Cheap -  Nothing is more cheaper as a device for voice calls
3. Reliable - powered by network and not by Home UPS/home socket/battery etc all of which fail more frequently. Requires little to no maintenance and is as reliable as the network is. For all power requirements of this network, all equipment in network is on 24x7 UPS, backed by DG supply if utility power fails an
4. Availability - All equipment inside our facility and operated by one provider. No dependency on network and events outside facility like pulling down of cellular towers, Cellular towers shutdown/maintenance, congestion of network, signal strength, Fiber/Cable cut of landline, etc

Many residents already have *spare fixed line handsets from other property, broadband service, etc.

Second, we have extension of residents apartment using hierarchical dial plan with 6 digits such as

Lotus 1101 - 601101

Lotus 101 - 600101

Mayflower Q-1001 -> 731001 [Tower(7) + Block(3=Q) + Flat No.(1001)]

Daffodils E-702 -> 350702

These numbers stay same as long as building stands. If a tenancy changes, the number does not [A big issue on subscriber owned mobiles and landlines].  

Third, because these numbers are easy to construct mentally just before dialling rather than remember or have the need to lookup in directory. Kids, Senior citizens, less educated helpers, contractors, security staff, etc can also use it apart from tech-savvy citizens from anywhere and everywhere where an extension is present.  The remaining 50-100 common area extensions are 3 digit numbers referable by directory lookup (hard-copy, PDF file, web-page, etc)  like any hospitality business and many of them maybe easy to remember. 

By contrast landline providers like BSNL and Airtel can only allocate 4 digits in centrex, which is insufficient atleast for APR with 8 towers, 30 blocks and 44-64 flats per block. This would require directly loading & maintenance of 1500-1600 numbers in all of the 1500-1600 terminal's directories, a job not easy to accomplish. BSNL and Airtel solution can work for APR Villa projects [Villas numbers ranging from 001-999] . Similar restrictions apply to cellular number allocations

Fourth,  deploy a Business Grade PBX [like Matrix Eternity Series] which can handle voice (and in future video), mobile terminals (like smart-phone with app tied to intercom and roaming inside facility and ringing in parallel with landline extension), can support supplementary service like call forward conditional/blind/always-disable to mobile, and can do automatic callback if a particular extension is busy [suitable for security, maintenance, club, etc]. Basically make the system smart and intelligent

If anyone has concern that such system cannot work well, please observe (and ask your IT/facility Administrator) what is deployed in your office as well as your previous or other BIG properties. You will know what industry trusts and whether we are going with the industry or against it.