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There is a simple calculator that will help you determine this: http://www.erlang.com/calculator/lipb/
You need to know something about the type of CODEC used for voice compression in your particular case (as noted in JimLynch's answer), as well as the packet frequency, so that the calculation will have validity. Hopefully this will help you ensure you have sufficient bandwidth for good voice quality. Be certain that you have enough bandwidth, because if you don't you will get packet loss, and as soon as that happens, your going to get IMs/emails/calls from everyone from the CEO to the janitor. Don't ask me how I know! Good luck!