My experience has been that the QoS mechanisms covered previously don’t provide a complete solution to the need for assured bandwidth when using VOIP over DSL. When the connection to the ISP becomes saturated for any reason VOIP traffic can be delayed which is always a problem. When managed QoS was combined with “traffic shaping” our VOIP phone service became much more reliable. This has proven to be true even on a very busy connection to my ISP.
Like the QoS mechanisms covered previously, traffic shaping is an edge process that occurs in your router. Traffic shaping is actually a process of reserving bandwidth specifically for selected applications. That bandwidth will not be used for other forms of internet access. As before, this tends to be most critical with outbound traffic where available bandwidth is most limited. It’s also true with inbound traffic, but this tends to be less of an issue.