Just for clarification: usual external modems do not prefer a specific type of computer or operation system. As long as a modem is properly connected (using the appropriate cables plugged into the appropriate connectors), all you need is some software using a command set compatible to the modem, and you might reach to everything you can dial up to. If you have a small soldering iron available you might look for a Mac RS422 to Modem RS232 cable description and build the cable you need. Do not become discouraged, the first time I wanted such cable it took months until I found every piece of information I needed. Today we've got intarwebs and this forum
Software for different purposes using modems is for example CoMa Mac (communication manager, including sophisticated answering machine features, customized versions might still be available directly from the developer), FirstClass (BBS software supporting dialup connections), ZTerm (Terminal, scriptable), Microphone (another Terminal software), FaxExpress Solo (Fax software with excellent self configuration capability), FaxSTF (another Fax software), FreePPP (connect to Internet, some additional software and configuration of your mac might be necessary).P.S.: Most likely you will need a separate power supply for the modem. Again: look for matching specification (voltage, polarity, power rating) of the modem power input and the supply power output, even if the connectors seem to fit. Also the cable to connect the modem to a phone line must have the correct pinout. Despite it is possible to use a foolproof pinout, some devices use a different cable... Take care.