I have internal Conexant HSF fax/modem and it works fine as modem - I can dial up with KPPP.
I installed Hylafax for S12 from slaki.eu and run faxsetup and faxaddmodem then /etc/rc.d/hylafax start

root@darkstar:/etc/rc.d# ./rc.hylafax start
HylaFAX: faxq hfaxd (without old protocol & without SNPP support).

then tried Kdeprintfax to send file. the log said:

Converting input files to PostScript

Mime type: text/plain
Command: enscript -M %psu -E -p %out %in
[ 1 pages * 1 copy
left in /tmp/kde-sfsynop/kdeprintfax_wJxk3ADa

Sending fax to 9733569 (pt)

Sending to fax using: /usr/bin/sendfax -n -h 'localhost' -m -x 'pt' -W '04624591' -s 'a4' -d 'pt'@'9733569' '/tmp/kde-sfsynop/kdeprintfax_wJxk3ADa'
request id is 6 (group id 6) for host localhost (1 file)


And that is all. No sound from the fax nothing at all. Can anybody tell me where to look for a clue? /var/spool/hylafax/status/ttySHSF0 says
"Waiting for modem to come ready" but KPPP said modem is already "ready". Is that a problem?

You don't have kppp running at the same time do you? That could create a problem (just guessing).

I'd check the Hylafax config files and make sure the init string is correct.

From http://www.linuxant.com/drivers/
Perhaps you need the full version?