[Asterisk-Users] Garbled voice on long distance calls

[Devon Stephens]

I've been having random problems when I make long distance calls using 
either VoicePulse or Nufone.  Sometimes the calls go through clear, and 
other calls (or even just part of a call) the person on the other end 
just hears garbled voice, or really broken up voice.  Sometimes it lasts 
for only a few seconds, but other times it goes on for a few minutes 
until I give up on the call.
At first I thought it was just a problem with our ISP, but I also have a 
Packet 8 phone that works just fine even when Asterisk is having these 
problems.
I get decent ping times to the Nufone or Voicepulse servers (85-200ms) 
and only minimal (0-2%) packet loss.
I've tried connecting with Ulaw, and G726(with Nufone), but it doesn't 
seem to make a difference which codec I connect with, I still get the 
same  garbled sound.
I'm have jitterbuffer=no in iax.conf, but I've also tried it with 
jitterbuffer on as well.
Local calls through the zap card seem just fine.
What could be causing these problems?