Krishna,
You’re going to need high speed internet access between these three centers and Illinois. We can push 100 simultaneous calls through a 1.5mbit connection so you wouldnt be looking at much bandwidth. What do you need at these centers?
Call between them all as extensions?
Be able to call out of each center as you would locally and not be charged long distance for the call?
Voicemail?
PBX functions?
Email Voicemail?
How many extensions per center?
Please be very specific in what you’re looking to do at these centers.