Different behaviour : command line / cron

Hi, I've developed a simple script that among other things sends a fax using hylafax's sendfax program. If I test it calling directly from the command line it works fine. If I let it run from cron it executes everything fine except the fax. I am using the system call and in order to debug I am using the following code ... $cmd = sprintf('/usr/bin/sendfax -n -d %s %s',$fax,$file) ; mail ("myemail@foo.com", "fax", $cmd."\n".system($cmd)); ... The output from the command line (/usr/bin/php /path/to/file/script.php) /usr/bin/sendfax -n -d fax_number /tmp/fax.txt request id is 471 (group id 471) for host localhost (1 file) The output when it executed from cron (*/1 * * * * /usr/bin/php /path/to/file/script.php) /usr/bin/sendfax -n -d fax_number /tmp/fax.txt In both cases the fax.txt has the same content. Any idea why and how to solve it?