EXPERT RESPONSE
This is going to vary widely from site to site. I would suggest that you look at the whitepaper "VoIP-Is your infrastructure ready?". It addresses several issues and will help explain how to look at your infrastructure. You can also hire a certified infrastructure auditor to assist.
When you are looking at VoIP and bandwidth, you should look at port utilization on all ports over a period of time. You will want to assume that the highest level of traffic is your constant level of traffic. VoIP is more real time than data. Therefore, it does not work well with retransmissions. Overall network health is extremely important and is often a reason that installations fail. In general, 64k per user is a guideline, but this will vary based on equipment, compression, etc. Your vendors can help. For instance, I know that Avaya will perform an assessment for you as part of their installation services.
Whatever you do, make sure that your assessments include the physical layer as well as the electronics layers. Echo is return loss and is usually attributed to poor cabling installation. Your cabling should be in good working order and pass all testing parameters for the category and/or class of cabling that was installed (the current test parameters).
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