asterisk users - Jan 2010 - send faxes as "3,1 kHz Audio"

Hi,

I have installed Asterisk with iaxmodem to send faxes with Hylafax.

But I have problems to send some faxes because the receiver does not accept
"speech". I must send the faxes as "3,1 kHz Audio"

But I do not find a possibility to do this. I need urgent help!

Chris