http://wiki.sangoma.com/wanpipe-linux-a ... -debugging
в случае нелинейного эха OSLEC вообще не помогает, вендор рекомендует регулировать txgain и rxgain..The other problem is that the non-linear processor is missing entirely from the s/w EC so that there is always residual echo that has to be manipulated with the gains.
.. Try decreasing your txgain and rxgain in zapata.conf or chan_dahdi.conf, and increase the volume on your handsets.
Нелинейное эхо: обычно возникает в случае шаманства у оператора - эходав на той стороне вносит свой "вклад".