For background information on various traffic analysis concepts and features that are applicable to Voice over IP (VoIP), see the white paper, Traffic Analysis for Voice over IP. This document presents fundamental traffic theory, several statistical traffic models, application of traffic analysis to VoIP networks, and an end-to-end traffic analysis example.
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Traffic Analysis for Voice over IP
http://www.cisco.com/en/US/tech/tk652/tk701/
technologies_white_paper09186a00800d6b74.shtml 
Use the Cisco CallManager Capacity Tool to calculate the minimum number of active subscribers that are required to support a given installation. Inputs consist primarily of quantity and usage information on the various device types that are supported in a Cisco Unified CallManager system.
•Cisco CallManager Capacity Tool (CCMCT)
http://www.cisco.com/cgi-bin/CT/CCMCT/ct.cgi
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