I have a small number of Grandstream GXP2000 IP Phones connected to a Trixbox on the LAN which connects via a Vigor2910Vg router to a Draytel SIP trunk over an Easynet 8MB/768K (5:1 contention) ADSL line.

On the Vigor2910VG I have disabled the onboard voip ports and set up QoS rules for the Trixbox and IP Phones. When calls are made, the QoS stats show that the QoS rules seem to be working, but the call quality is awful - constantly breaking up.

If I make a seperate LAN by adding a second router (Linksys) to the same ADSL line on a seperate public IP address (not via the Vigor2910Vg) and then connect the Trixbox Server and the IP phones via the second router, the call quality is consistently fine using G.711A or G.729A with no choppiness or drop-outs, so the ADSL line is obviously not the problem.

I guess the problem is either with LAN traffic behind the Vigor2910Vg router or a problem with the router itself .

Would anyone care to share their thoughts/experiences ?

I'm sure I read somewhere that for Grandstream phones and Draytek routers, the Diff-serv layer 3 QOS / Precedence value should be changed to 46 on the advanced screen of the phones web interface.