I provide extensive troubleshooting (including hardware and software versions) about this problem in the following forum post:
http://forums.digium.com/viewtopic.php?p=125244#125244
We’re starting to think that there’s some kind of timing issue going on here that is introducing noise on our PRI lines. We’ve recently upgraded from Zaptel to DAHDI, (without upgrading our Asterisk version) at which time the problem was introduced. Perhaps I have set up chan_dahdi.conf wrong? We’re using Bell Canada for our PRI provider, and without further ado, here’s our chan_dahdi.conf:
[channels]
usecallerid=yes
hidecallerid=no
callwaiting=no
threewaycalling=yes
transfer=yes
echocancel=yes
echotraining=yes
echocancelwhenbridged=no
immediate=no
group=1
signalling=pri_cpe
switchtype=5ess
context=local
channel=>1-23
rxgain=0
txgain=0