http://www.digium.com/en/products/software/digium-phone-module-for-asterisk

http://www.digium.com/en/products/software/fax-for-asterisk

https://store.digium.com/cart.php?action=add&product_code=816-00003&quantity=1

http://my.digium.com/en/docs/FAX/fax_faq/