I have an intermittent echo with my system that utilizes a PRI circuit. I believe the issue is with certain channels on the PRI circuit that when used, the echo is heard. In order to pursue this theory I would like to compare the list of my troubled calls against the system log. But looking at the log I cannot tell which PRI channel is used for each call. As an example, the system identifies the channel used to be DAHDI/i1/18005551212-927. The DAHDI/i1/ part is the same for all calls. After that, the phone numbers obviously differ, then I have a hyphen and a 3 digit number. I am not sure if or how the 3 digit number is translated to a PRI channel.
Since asterisk 1.8 that is the new behavior i1 means span 1, for good reason,ISDN calls are not necessarily locked to a bearer channel as the D channel might put it on hold or redirect
It is very unlikely that there are variations in your b channels , as they are TDM, unless of course a channel bank is involved. It is highly unlikely that your provider is using one.
If indeed a channel bank is involved (yours) then more traditional telephony tools might be needed at the FXO/FXS level
there are however a couple of new variables, namely ${CHANNEL(dahdi_span)} and ${CHANNEL(dahdi_channel)} that might help you debug if you want to customize the FreePBX dialplan.
You can still use dahdi_monitor at the bearer channel level for echo/level monotoring and if you suspect certain b channels then that might be the way to go.