

You shouldn't have ignored the ZyXEL.  It can be purchased with a "Mac
bundle", which includes a hardware-handshaking cable and FaxSTF software.
The bundle adds between $35 and $60 to the price of the modem, depending
on the supplier.  It is true that the modem has no Mac-specific docs,
but it doesn't require much 'tweaking' (aside from setting &D0 in the
init string, to enable hardware handshaking).

For more information on the ZyXEL, including sources, look at various files 
on sumex-aim.stanford.edu, in info-mac/report.