Fax machines transmit audio tones with the data encoded on it, much like the old dial up modems. VOIP chops up the audio into packets, which are then re-assembled at the far end.
There is no continuity, so the fax will not work, the same as a modem would not work. This process does not affect speech, as there are tiny pauses in speak.
It's one of the ways I communicate with my 88 year old deaf mother. She can text but for longer messages she faxes me. Working on facetime to sign but proving a challenge!
Wouldn't iMessages be a better alternative to Fax whilst you're learning sign language?
The messages can be typed, dictated or, on some devices, even handwritten (on screen) and are sent as text
Sign language is not the issue (first language for both of us!) - just getting over the technical difficulty for someone who has never used a computer is the issue.
I suspect I will retain the copper though an expensive option for 2 or 3 faxes a month.