Fax for Asterisk requires purchasing a license from Digium.
Once you have the license:
- Follow the license and module installation instructions at: http://downloads.digium.com/pub/telephony/fax/README
- Set system configuration "Fax send driver" to "digium".
- Set system configuration "Fax receive driver" to "digium".
- In each desired role set "Send faxes" to "Yes".
- In /etc/asterisk/modules.conf under the section [modules] add the line:
noload => res_fax_spandsp.so
t38pt_usertpsource = yes