Hello Mychaela,
there's no really good option for this right now (especially at higher
speeds), but supersat wrote this yate module to run an old "Winmodem"
driver binary (sl-modem) from a PBX.
It's not very polished, but it will work up to 33.6k (or even 56k in
outgoing calls).
There are some problems/concerns regarding reliability of the
connection, I still haven't gotten it to be fully stable.
https://github.com/Shadytel/shadysoftmodem
Good luck!
Best regards
Sarah / manawyrm