This article is from the Mac communications FAQ, by Bruce L Grubb BruceG6069@aol.com with numerous contributions by others.
Any *external* Hayes compatible modem will work with your Macintosh.
There are too many to list or review here. The USENET newsgroup
comp.dcom.modems is a good place to ask questions about the many
different external Hayes compatible modems. Such modems can be used
with any computer (Macintosh, UNIX box, MS-DOS PC, Amiga, etc) with
a serial port (e.g.: Macintosh modem port) or USB interface.
However, there *are* modems that are designed specifically for use
with the Macintosh but they are usially internal modems.
Note that modems for the Macintosh Performa apparently can only
plug into the Macintosh Performa because of an extra pin which
they posesses (there is a corresponding extra hole on the
Performa serial port connector into which this pin fits). As a
result, the bundled Global Village FAX modem can only plug into
the Performa. Other modems can of course also plug into the
Performa; the extra pin only prevents plugging the Performa-specific
modem into other Macs.
Assuming you wish to use an external modem, your only other hardware
consideration is to find an appropriate hardware-handshaking cable
to connect it to your Macintosh. (see [1.2] for details).
Various special modems exist with unique features; the most
notable type is the dual FAX/modem. For more information,
visit comp.dcom.modems or your local dealer: there are simply
too many products to describe here.