There won't be one.

Echo is an audible representation of a problem with an analogue part of the call (typically analogue circuits outside the callmanager system, or produced from the handset/speakerphone at either end). Routers have echo cancellers that can determine what is echo, and suppress it - if it isn't suppressed, it's becuase the system doesn't realise it is echo.

In this case the system wouldn't know to log it either.

Best thing to do is to get it logged better when it happens - i.e. get users to not calling/called number, time, etc so you can trace the call and see if it's reproducible.

Aaron