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It sounds like you have already identified the cause of your VoIP quality issues - a lot of people simultaneously on the network using relatively bandwidth intensive programs as they stream music/video. What you might want to look into is how to control network traffic so that you can prioritize VoIP over non-crucial use. You might want to explore Quality of Service (QoS) as a way to make sure that packets dedicated to VoIP become the preferred packets. It depends on the router, but there may be options in the setup software where you can switch on QoS and choose to give priority to your VoIP software/system.