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

• Ensure that /etc/asterisk/sip.conf under the section [general] has the line:

t38pt_usertpsource = yes