Efficient network setup can enhance the quality of your VoIP experience. Even if you are averse to VoIP terminations for routing long distance traffic, within your call center, you can take advantage of VoIP. Here is a simple but efficient topology for Asterisk based call center setup.
Here all the agents, supervisor and admin phones are within the LAN and have assured bandwidth. Good quality IP phones from Polycom, SNOM, Linksys and other vendors can serve as the admin phones. Agents can use softphones like X-Lite or Eyebeam. An enterprise grade call center software like Indosoft Q-Suite will provide all the necessary features to setup and manage an advanced contact center. I recommend DELL 2950 category of servers which has served as very well in the past. The Dell web managed switches are inexpensive and work well within the contact centers. We implemented our first VoIP based contact center using Asterisk in 2003 with the long distance traffic terminated on PRI using Digium TDM board.