Dear Hafeez,
I am IT manager of a US based medical billing company of NJ. I sit in Islamabad and I have already established this type of call center here in islamabad. All you need is a gateway for VoIP. We have used multitech gateways. One you need in USA where you will put normal phone lines into it. Another gateway in Pakistan where you will need to put telephone sets or PABX exchange. both gateways will need static IP addresses. The license is free. There are no hidden charges. However you cannot terminate your VoIP call in PTCL exchange because its illegal. If you want to use sattellite upload than you will have to get a free NOC from Pakistan Telecommunication Authority.
I will be more than happy to help you out in case needed.
Best Regards,
Aksam Zaheer Dar