For background information on various traffic analysis concepts and features that are applicable to Voice over IP (VoIP), see the following white paper:
•
Traffic Analysis for Voice over IP
http://www.cisco.com/en/US/tech/tk652/tk701/technologies_
white_paper09186a00800d6b74.shtml 
See the following resources and best practices for sizing contact center components based on the amount of traffic:
•Cisco CallManager Capacity Tool
http://www.cisco.com/cgi-bin/CT/CCMCT/ct.cgi 
Use the Cisco CallManager Capacity Tool to calculate the minimum number of active subscribers that are required to support a given installation. Input consists primarly of quantity and usage information on the various device types that are supported in a Cisco Unified CallManager system.
•Bandwidth calculators for contact center systems
http://www.cisco.com/univercd/cc/td/doc/product/icm/bandcalc/index.htm 
The output from these tools can also be used as input to the IPCC Express Configuration Tool and the Cisco Unified CallManager Capacity Tool.
In addition, these third-party traffic engineering tools are provided for your reference:
•VoIP Bandwidth Calculator
http://www.packetizer.com/voip/diagnostics/bandcalc.html
•Online Erlang traffic calculators
http://www.erlang.com/calculator/voip_calculator.htm
The following SRND also includes information on sizing your contact center components:
•Cisco IP Contact Center Enterprise Edition Release 7.0 Solution Reference Network Design (SRND)