I have found this to be a much more difficult problem than you may first
imagine. I have still not been able to get my traffic shaper totally
under control with DSL.
In a nutshell, you have to be very careful that your traffic does not
get queued by your DSL modem.
The best I have been able to figure out is that this is only possible by
severely limiting your upload and download speeds. What happens is that
your upload gets maxed out, the modem (or ISP) starts to queue stuff,
and it is delayed too long for VOIP.
I think this is one of the materials I read that helped me understand a
little better -
At one point I had ping times across my VPN from CA to PA of over 3
seconds - until I turned off the traffic shaper!
As mentioned by another poster, a codec that uses less bandwidth is
probably also a good choice for you and your limited upload speed.
Hope this helps,