Hi Shakthi777, just offering you some ideas.
Since I live in Australia
here's a couple I sourced for you.
The comparison between hosted PBX services and premise-based PBX
http://articles.directorym.com/Hosted_Vs_Premises_VoIP-a852309.htmlhttp://www.planetisp.com.au/pabx.phphttp://www.ringcentral.com/