To verify that QoS will solve the problem, temporarily disable wireless, and disconnect all devices from your SBG6580 except your VoIP equipment. Make a test call. If the problem is solved, then QoS will indeed solve the problem permanently. If you still experience issues, the problem is caused by something else.
As far as I'm aware your modem/router does not support QoS. If you get another router, I recommend one that is compatible with the Toastman build of Tomato firmware, such as the Asus RT-N16 or RT-N66U if you can afford it.
If you use another router, Google for how to place your SBG6580 into "bridge mode" so that your new router is assigned a public IP address.