(excerpt from HylaFAX FAQ, http://www.vix.com/hylafax/FAQ/Q1.html)

     HylaFAX is a facsimile system for UNIX systems. It supports:
        o sending facsimile
        o receiving facsimile
        o polled retrieval of facsimile
        o transparent shared data use of the modem
        o sending messages to pagers
     Facsimile can be any size (e.g. A4, B4), either 98 or 196 lpi, and
     transmitted/received as either 1D-encoded or 2D-encoded facsimile data
     (2D-encoded data is frequently more compact and hence takes a shorter
     time to communicate). Any modem that supports one of the standard
     interfaces for facsimile operation can be used; i.e. any Class 1, Class
     2, or Class 2.0 modem.