Intercoms are provided in gated societies due to security reasons so that stop people into any gated society without checking their identity and intercom play an important role, by which you can allow entry of people and validate their identity. By using intercom residents can interact by other residents, maintenance department, security guard etc. An intercom system or PBX (Private branch exchange) is usually installed by the builder before occupation or sometime a community may install this system.
It supports only limited number of connections. As per your query in your building intercom connection was for only 170 flats. And all connections are occupied and no slot is left. So in this case if intercom connection is required for 5 more flats, first of all it is to check whether any connection is vacant or not in use. Otherwise the whole intercom system including base station is to be upgraded. Further, if there are more flat are left, other than 5 flats which you have mentioned and which will require intercom in future then a new intercom may be installed in addition to current intercom system. But of course it will cost additional charges.
A base station costs around Rs.50,000/--1,00,000/-, Installation cost will be approximately up to Rs.1,00,000/- and Annual maintenance costs will be around Rs.20,000/- to Rs. 25,000/- per month.