I also use vonage and had similiar issues. To fix them we delved into the vonage box and gave it a static IP that wasn't assigned by our router. Next turn on DMZ Host for that specific static IP address. Then we also turned on QOS to highest level for the port it was plugged into or you can do it by IP depending upon your router. QOS, DMZ host, and a static IP like that fixed all of our problems with quality. Now we get excellent call quality even while downloading at 1.5 Mb/s and uploading at 95Mb/s (nearly max for my house's connection).
"Catch a wave and you're sitting on top of the world." - The Beach Boys