spandsp implements only unencumbered standards, so it may be distributed freely. The higher speed modem for FAX, V.34, is encumbered by patents at this time. Surprisingly few FAX machines and FAX modems support V.34, and the coverage is patchy. For example Brother's cheapest multi-function machine supports V.34, but few of their more expensive machines do. Unless a lot of your FAX traffic is with companies using high-end FAX machines, you problem won't miss the higher speeds very much.