Asterisk echocanceller

My box is asterisk 11.10, dahdi 2.6.8 with oslec into debian wheezy and connected to telco by rseries digium box.
My situation :
Echo occurrence is rare but persists.
Echocanceller value is 256 taps (the best I’ve had).
I used dahdi_monitor with the -r option and I have the echo in recorded file.
I concluded that oslec can not remove the echo.
Are there other solution for software echocanceller or I have to buy cards with hardware echocanceller?
Any idea ?

That’s the best software echo canceler available. So, if it’s not working, you should consider the hardware cancelers instead.