Since correct modem operation on a NeXT depends upon a correctly wired modem cable, buying a Mac cable is not a good idea. Some Mac cables do not allow dial-in and no Mac cable allows the use of hardware flow control. For these reasons, we are recommending that only cables that meet NeXT specifications be used. These cables are available commercially from NextConnection, and from Computer Cables and Devices, or can be custom built. Hardware flow control is absolutely essential for all serial port connections with speeds of 9600 bps and above.
Most people use tip or kermit to control the modem.
This document is available from most FTP sites that carry NextAnswers. In order to use a fax modem with the NeXT Computer, a NeXT compatible fax driver must be available to operate the modem. Other fax modems must supply a NeXT compatible driver. After installing a fax modem using PrintManager one must repeat setting things in the Fax Options panel in order for them to be stored correctly. In particular, these include the Rings to Answer and Number of Times to Retry.