Echocancel and faxdetect

I am using asterisk 1.2.13 with a TDM400 card.

When I set faxdetect=incoming and echocancel=true
in my zapata.conf, if I receive a fax, asterisk disables
the echo canceller and the fax is received properly.

But when I set faxdetect=both and echocancel=true,
if I try to send a fax, asterisk doesn’t disable the echo
canceller and many faxes cannot be sent.

Are these behaviors with echo canceller as expected ?

Any Idea?