Asterisk based new Voip Implementation- help needed

hi all,

Well our company is planning for VoIP implementation.we have two options at our hand

  1. Impement VoIP using Cisco UCS
  2. Implement VoIP using Asterisk

well we have decided to try asterisk as the Implementation cost is very less compared to Cisco’s VoIP solution.
here is our setup
we have our offices in US,UK,& India
All the locations are connected through MPLS
we have decided to install 3 Asterisk servers ,one in each location and connect them through MPLS.
For a user in India , All the calls made within India should be a local one, If they try to dial US number it should be routed through US asterisk hence again a local call and to UK its through UK asterisk and again its a local call and viceversa.

Somebody please help me out in this by providing the inbound & outbound dial plan.
What service should be subscribed for outbound calls? i.e like ISDN or PSTN
What is the hardware requirement to implement the same.

Please friends,…help me out in this as I have to submit my proposal…

There are lots that you need to know.
First, how many concurrent calls will you be expecting the systems to handle?
How many users will each site have? What kind of phone will you use? Are you planning on using a digital circuit to interface to the phone company or pots lines? Then you can figure out the equipment you will need.