IAXmodem is a software modem written in C that uses an IAX channel (commonly provided by an Asterisk PBX system) instead of a traditional phone line and uses a DSP library instead of DSP hardware chipsets.

IAXmodem was originally conceived to function as a fax modem usable with HylaFAX, and it does that well. However IAXmodem also has been known to function with mgetty+sendfax and efax.