[asterisk-users] Help - Poor Voice Quality

[Jim Duda]

I'm struggling to get my VOIP installation to be acceptable.  I'm 
looking for advice on what else I can look for.

My system:
o Teliax VOIP service, voip-ny1 proxy
o RCN Cable Internet Service (3Mbps download, 500kbps upload, 6ms 
average jitter)
o 3.2 GHZ P4 Server (runs asterisk, firewall, other stuff)
o server lightly loaded
o Linux kernel 2.6.19.2
o Shorewall Firewall software with QOS configured for VOIP P1
o Asterisk 1.4.0
o Sipura SPA-2000
o Grandstream GXP-2000
o IAX connection to teliax

Outbound voice quality is many times horrible, to the point where ppl 
say they cannot hear me.  The voice often drops out.  Inbound quality 
seems to cut in and out too.

I downloaded the myVoipSpeed VOIP analyzer.  It indicates that I have 
plenty of download and upload bandwidth.  I also have good jitter.  The 
tool doeesn't find any packet loss whatsoever.

My RCN cable company cannot find anything wrong with my cable modem.  No 
packet loss.  I'm supposed to be paying for 10M bit downloads, but only 
getting 3M bit.

I've been on the shorewall firewall and confirmed that I have the 
firewall configured properly for VOIP QOS.

I'm using the basic asterisk iax.conf setup with only those changes 
required to interface with the teliax service.

I have the same issues with both the Sipura Adapter and the Grandstream 
phones, however, I do believe the Grandstream appears worse at times.

I've attempted to analyze the IAX traffic using the Wireshark ethernet 
protocol analyzer.  Everything looks okay best I can tell.

What else can I do to analyze why the voice quality is so bad?
What can I do in Asterisk to help track down where the problem is?

I want to make this VOIP work.

Thanks for any help.

Jim