Spandsp contains a complex T.31 (class 1 FAX) modem. There are several options for using this with packages like HylaFAX. IAXmodem is currently the most popular for Asterisk users. It provides an IAX interface on one side, and a series of pseudo-tty devices on the other. Freeswitch has T.31 modem support built in, presenting a series of pseudo-tty devices to a FAX package. HylaFAX, eFAX and other similar packages can attach to the psuedo-tty interfaces.