Hey… This Is Reserved For My 3rd Year Project… 
Making Ur Own PBX System… Without A PBX Machine…!!!
Check It Out Its’ Cool, Its’ [Almost] Free…
Tutorial + Links
Now I Can’t Say That Its’ Worth A Try, But Ya Its’ Worth A Look,.. 
Well that is a incredible idea you have… But i think that is to hard to do…
you can also check Fonality trixbox CE, an Asterisk-based PBX Phone System (formerly Asterisk@Home) | trixbox
I have found this software powering a famous telecomm. based service in mumbai., its also based on asterisk.
please check the link once more
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