shell_exec problem

Hi everybody I have some few problems with the shell_exec command. I want to send a fax with hylafax using the sendfax program in linux. When I try to do shell_exec ("sendfax -n -d faxnumber faxfile") it will not work I can see in my httpd file that linux only gets the command 'sendfax' and not the parameters '-n -d .....'. Can anybody help me? Thanks in advance Coert Metz