I strongly doubt that it would be possible over IP (FoIP? Fax over IP?) since VoIP calls are compressed.

AFAIK, GSM communications are lossy too, so not suitable for faxing.

If I'm wrong, hylafax is available for armel architecture in debian, so it shouldn't be too hard to make it work on the N900.

or you could setup a email-to-fax gateway at home (given that you have a mail server around...), i wrote a guide for doing this with hylafax some years ago.