What are the alternatives to the sub-committee proposal ?

SHORT ANSWER:
Centrex (PBX in Service provider network), Mobile-Mobile P2P calling, Mobile CUG, Building Intercom System. and  an On-Premise PBX solution.

Here is a quick comparison summary of these solutions stack up:

Alternative Vs Competency	Big Directory Lookup	Cost Competitiveness	Service Reliability	Smart PBX features
Centrex	Yes	Worse (Indirect)	High	Very Good
On-Premise PBX	No	Medium	Highest	Best
Mobile CUG	Yes/No	Medium	Low	None
Mobile P2P	Yes	Best	Low	None
Building Intercom	No	Worst	Highest	None

From a pure solution competency and fitment competency angle, you can see why the sub-committee prefers the on-Premise PBX solution. 

DETAILED ANSWER:
Here are what we could think as potential solutions to the facility communications problem and not limited to few areas like visitor management (guest, e-commerce delivery, service of household goods, etc)

1. Centrex (PBX in service provider network ) 
2. PBX (on-premises) and Analog phone (Intercom solution) 
3. Mobile-Mobile Closed User Group (CUG) 
4. Mobile calling or P2P mobile calls
5. Dedicated Building Intercom systems 

(1) and (2) are proper intercom solutions which solutions that can satisfy most of the sub-committee thinking and requirements, while (3) and (4) are just attempts to mimic intercom functionality when no PBX system is available, infeasible to implement, etc. (5) is a solution more targeted to visitor management application rather than generic intra-facility communications. 

(1) and (2) are basically the same type of solution, the only major differentiation between whether the PBX is inside the facility or outside in service provider network. Centrex has many advantages for residents as it requires ALL residents to just have a landline from the provider (Operator A in our case), and then he adds all users to a centrex group and provides PBX like features to calls made between members of a centrex group including unlimited minutes of use. The operator does maintenance of equipment and phones. The disadvantage is that in India only 4 digits can be used to specify the flat no and remaining 4 is allocated by operator. For eg. 4130 can be the common 4 digits of centrex number with the last 4 digits allocated  to the facility to decide. For APR condominiums 4 digits are insufficient to specify a flat uniquely as our flat numbers are non-unique 4 digits (10th and 11  floor itself) and we have grouping of flats in tower and block with no way to specify those. Therefore this requires maintenance of directory of 1500-1600 extensions which is impractical.  Plus it forces everyone to use a broadband connection from one operator destroying the entire concept of choice of broadband provider.

An On-premise Business-Grade PBX on other hand puts a telephone exchange inside the facility, creates an independent telephone network and delivers voice (and other) services on that via the simplest to use, cheapest to buy and most commonly available traditional analog phone. you can configure services, do forwarding to mobile, create any dial plan which maps unit nos. to intercom no., etc, provides unlimited minutes for internal communications. It can be used to make outgoing calls which are billed only to the residents who use the feature. And it reaches everywhere we want a communication point in the campus. 

A Mobile Closed User Group (CUG) is very similar to Centrex, minus the PBX features and done using cellular phones rather than landline analog phones. the benefit is no charge for calls made between users of a mobile CUG (same as the above two features). It depends on availability of good cellular signal and less congestion at all signal to work reliably and no congestion of calls made to specific number. It is usually deployed for mobile teams of government and private agencies who roam outside campus but need to continuously communicate with members of their organisation. More Point-to-Point communications unlike PBX and Centrex which work better for Multi-point-to-Point call traffic. it also requires society to buy 1500-1600 cellular numbers with 4 digits allocate to operator and circle, and the remaining 6 potentially usable to describe each extension, provided operator can allocate. 

Building Intercom system requires big investment in apartment infrastructure, may not  retrofit with homes easily as builder has not planned wiring for it during construction nor promised it as deliverable. Its terminals are usually video enabled and their a big upfront cost in terminals and wiring infrastructure development.

Mobile P2P method requires zero infrastructure setup. Just provide mobiles to each common area location. Residents already have mobiles. The trouble is most high rise with basements (have spotty mobile coverage and absent signal in basement where many facilities and offices are located) making calling unreliable or impossible in many scenarios. Directory management is a big pain and residents keep changing due to tenancy and synchronisation of contacts is as big issue. Then their is a question of misuse. Security stats using visitor mobiles and ask them to directly call residents and handover phone to them to confirm if the number with them is unanswered, not reachable, wrong, etc. They really have no idea whom they are speaking to, comprising the apartment security. This is the biggest flaw of mobile P2P calling as observed from current use and deployment.