What is Centrex Facilty on BSNL FTTH service / plan ?

HI ..

I was looking for information about bsnl fibre cable internet services and everything was clear what was Centrex Facility on it I was not able to understand. They if size of Centrex is less than 50 then Rs. 50 and if greater than 50 then no charges and all intra circle centrex calls are free ?

What is that ?

Also they say ..
Rent free incoming connections for Centrex group on FTTH to Corporate Customers /Multistory complex /Housing societies etc. may be provided for the use of Incoming calls and making calls within Centrex numbers only..

What is Centrex ?

It is central office based communication service, which integrates all your multi located telephone lines (Existing and New) into a single highly functional communication group with more distinctive features without any additional equipment (like EPABX) at your premises. Highly cost effective, Free Intra Centrex calls, No worry to select innovative (PBXs) equipment, No risk of obsolescence of technology, No requirement of power supply, No need to waste valuable floor space, No annual maintenance charges, totally flexible are some of the salient features of Centrex facility.
http://www.bsnl.co.in/opencms/bsnl/B...e/centrex.html