linmodem is a project originally created by Fabrice Bellard to implement a software defined modem to be used with a variety of "Winmodem" chipsets that became popular in the late 1990ies. Those chipsets are basically a soundcard attached to a telephone line, with all of the actual modulation/demodulation implemented as digital math on the main CPU of your computer. See https://bellard.org/linmodem/ for the original project homepage.
In Osmocom Retro-Networking, we are planning to modify and extend this code base to turn it into a SIP UA. See our Issue tracker for what is planned.
Status: At this point (December 2019), it is still vapourware.