Here are the two scenarios on how I plan to use Vonage VOIP service.

1) since this adapter is connected to "broadband internet" I assume
that I can physically place it any where in the world . now I think my
family in India can call me or anywhere in USA with this service also
my family will receive a US number so that any one from USA can call
them using US long distance.
2) Is there any way that multiple family members from India but at
different locations can make calls through same Vonage adapter to USA.
i.e. calling local number but routed through VOIP adapter.

Scenario one looks very much feasible to me. Just need your comments
to revalidate my claim.
Scenario two I need suggestions on how to setup this system. (EPABX ? ).
My budget for the entire system (Excluding VONAGE)is about 400 USD

1. yes.  In fact, you can order more than one adapter for extra monthly charges.
(one adapter in US and one in India)

2. yes--use conference call.  You just have to have the VOIP phone to
initiate all calls and connect them up.

CON: Don't count on great sound, especially bad echos can be very
annoying.  There are multiple companies but I've heard their sound
qualities are about the same.  I currently have Broadvox Direct and
have been ambivalent about keeping the service for this reason.  Lingo
offers the cheapest rate at $19.99, I believe.  If your relative in
India have computers and internet access, how about Skype?  Can't beat
free calls!

Thanks mewtwo-ga!
2. Can this conference call be initiated without manual intervention?
   do I need special phone or equipment ?
Skype is best but calling To and From PSTN telephone provides more flexibility
than using computer for making calls. Also getting my family US number
will be a great advantage!

For (1), technically no problem unless Vonage have some reason for
prohibiting it. It's a fairly standard use of VoIP.

I'm not sure you could use your Vonage adapter for (2). What you want
is some way of connecting your landline with an incoming call to VoIP.
The Vonage adapter is probably intended to connect a telephone handset
to VoIP. You can get other VoIP adapters which will interface between
a landline and VoIP but a cost effective way might be using Asterisk,
which is a software PBX running on Linux. You would need a telephony
card in the PC to link the PC to the phone line (you can also get
telephony cards to allow you to plug your handsets into the PC). The
PC can then be set up to answer an incoming call, ask the caller for a
security code and the number required, and then make an outgoing call
over VoIP. I'm assuming Vonage uses standard VoIP protocols, which
Asterisk will be able to mimic.

Asterisk is at
http://www.asterisk.org/

There's a wiki entry about using it at
http://www.voip-info.org/wiki-Asterisk

Owain

Hello Owain your comments kind of answers my question!
But I was wondering if there are any ?of the shelf? Hardware products
available to do the similar job.
Asking the caller for a security code before proceeding further is
nice but since I want this
Just for my family a simple hardware solution also will do. If I could
not get inexpensive ?Unattended Conference calling? device probably I
will end up implementing asterisk .

Thank you
Abhijit

I have been pointed (by a posting from Rob on news:uk.telecom) to the following:

... Linux distribution called Xorcom Rapid (http://www.xorcom.com/).  
They released a new version last week.  You will need a "spare" PC 
to set it up on as it formats the drive.  The handy thing is that 
after installing, it takes literally 10 minutes to have it up and
running with VoIP.  If you add the hardware telephone line interfaces 
it may take longer ...

Owain