FreelyCall supports T.38 and G711 Audio mode fax.

Fax with Fax Machine
If you are using a fax machine which supports T.38 or a classical fax machine connected through a SPA3102 to FreelyCall, you should have no issue sending faxes.

Fax with Asterisk
If you are using a PBX such as asterisk, here’s a few steps to help you get started sending faxes from asterisk through your FreelyCall trunk.

1- get a ‘free fax for asterisk’ license for free from digium here: http://www.digium.com/en/products/software/fax-for-asterisk
2- you will receive 2 emails from the digium, the instructions in the 2nd point to this URL: http://downloads.digium.com/pub/telephony/fax/README just follow exactly what it tells you, and you should have fax sending working
3- the .tif file to use should be fax compatible. the best way to create one is to use ghostview, passing along a pdf which will output the tif.

gswin64c.exe -q -sDEVICE=tiffg3 -r204x98 -dBATCH -dPDFFitPage -dNOPAUSE -sOutputFile=faxtest.tif faxtest.pdf

4- to test from asterisk CLI without creating a callfile, try:
originate SIP/phone-number@freelycall application sendfax /root/faxtest.tif,fzd

Once above is working, you can choose from a wide range of applications to send/receive faxes from your desktop.