lammert - 7:12 pm on Oct 29, 2010 (gmt 0)
It is not only bandwidth usage which you should look at, but also packet priority. It could be that the small UDP packets for DNS are processed at high priority, causing your voip packets to delay.
You should look in the manual of your Internet router to see if it supports QoS (Quality of Service). If it does, there are options to give voip packets high priority and dedicated bandwidth resulting in high quality calls, Independent on the amount of other packets flowing to and from the Internet.