Fabrice Bellard, because of course he did :), implemented his own version all the way to V.34 with parts of V.90 https://bellard.org/linmodem/ Currently actively maintained by Osmocom project https://osmocom.org/projects/linmodem "In Late 2021 we started a project to support AMR_Modem_Riser cards via a custom break-out board (#5294) and an associated FPGA softcore to interface the AC97 master interface with USB (audio)."
Tony Fisher wrote V.32 soft modem stack for SGI Indy back in 1996. York University couldnt handle hosting whole 8 megabytes of his stuff https://www.cs.york.ac.uk/news/fisher/home-page but at least they had the decency to upload it to github instead of just deleting https://github.com/university-of-york/cs-www-users-fisher/tr... Randy Rossi rewrote this project and got working V.21/23 https://github.com/randyrossi/fisher-modem
Fabrice Bellard, because of course he did :), implemented his own version all the way to V.34 with parts of V.90 https://bellard.org/linmodem/ Currently actively maintained by Osmocom project https://osmocom.org/projects/linmodem "In Late 2021 we started a project to support AMR_Modem_Riser cards via a custom break-out board (#5294) and an associated FPGA softcore to interface the AC97 master interface with USB (audio)."
V.21 modem implemented in Gnu Radio https://web.archive.org/web/20170627151353/http://aaronscher...
amodem - good source of ready made modulation implementations https://github.com/romanz/amodem
minimodem- simple radio link FSK modem https://github.com/kamalmostafa/minimodem
audsl - leased line project, not compatible with standard modems https://www.araneus.fi/audsl/ but might give some ideas.