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 Cisco Unified Contact Center 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 Unified Contact Center Enterprise Releases 7.0 and 7.1 SRND