Calling Card Unlimited Calling and info

Calling card set up for one of the Top American Company. We have a branch in New Zealand and it will be a scratch calling card where the customer will use the DID number to call followed by the pin number from using the land line and to set up the toll free number where the customer can use the Mobile to ring but get a call back on customers mobile. Apart from the setup , I would like you to find the top and cheap provider with the DID and termination rates. I would like to have a concept of unlimited calling and how this trick works. I would like to have unlimited calling to India for example and want to know the trick behind how the people does that. The setup will be purely asterisk base on Linux. Only the experience person who can offer us the contacts details and how the unlimited calling works specially to India, If you dont know how this works then please do not bid Thanks