Actually there are two concepts of Call Center in Pakistan.
1) Call Center for individual organisations like Banks.
This setup requires No VOIP or bandwidth . In that setup,there is one Server and multiple Agents sitting with telephone sets and PC.
When Call comes in, it can be routed to any available agent with call information on Agent’s Pc screen.
2) VOIP call centers.
In that setup:
At USA: You require VOIP Gateway, Gatekeeper (Software which controls calls and perform metering)
You require internet connection with bandwidth about 13k per Call.
And IP phones with PC.
Now Call from USA will be routed by Gatekeeper to your IP phone in Pakistan along with Call information.