asterisk users - Aug 2007 - how to specify a channel inside txfax command

Hi,
I've compiled rxfax and txfax on my Asterisk box.
I've made a small extensions.conf for test so that when I call a number 
with my Idefisk softphone I activate txfax command.
My goal is to try to send a fax using an analog line first and then an 
ISDN line but I do not know how to specify the channel inside txfax command.
Sorry if it may sound a stoopid question but found no docs on internet 
and "show application txfax" shows nothing useful.
What am I doing wrong?

TIA

Giorgio