If all the calls are from one point to another, you could save a lot by using Quintum and the PAcket Saver software.
That option allows to save bandwith on calls that are routed betwen two points by combining the overhead of each conversation in a unique link.
Being very conservative, without using PAcket Saver, you will be looking betwen 15-20 kb per active call on the system.
It will take a whole E1 worth of bandwith (2 MB) to handle all those calls safely and reliable.
With packet saver, and point to point configuration, you may save up to 30% of that bandwith.
Since it will be hard to find a E1 worth of internet bandwith in India, you may need several small gateways (like 24 port gateways), running on several internet links,like DSL.