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It doesn't take all that much bandwidth for VoIP calls, and more often than not, I have found the problem lies elsewhere when there are call quality issues. Basically, once you have allocated sufficient bandwidth for each call, you should look at jitter, latency and packet loss. Excessive jitter creates problems with echo cancelers and can cause echoes and talk over. Latency can cause momentary silence during calls, which often results in people talking over each other (then pausing, then talking over each other again, ad infinitum). Excessive packet loss will cause voice to be distorted and/or sound "choppy".