sendfax are run "as is", they will use their
compiled-in defaults from policy.h.
If the configuration files mgetty.config (see runtime-mgetty) and sendfax.config (see runtime-sendfax) exist (and are readable), both programs will get their run-time configuration from there. Items not specified there will still be taken from the compiled-in defaults. Command line switches will always override those settings. The configuration files are usually located in /usr/local/etc/mgetty+sendfax/.
If you specify command line arguments (see the mgetty(1) and sendfax(8) man pages for details), this will override both compiled-in and config file defaults.