Years ago, some poor and forgotten soul ran into this problem with a LaserWriter:
I am using a LaserWriter 16/600 PS with the fax option, and another standard fax machine connected to the same phone line. Even though the printer fax option is set to answer after eight (8) rings and the fax machine is set for two (2) rings the fax printer connects as soon as the standard fax machine answers the incoming call. It sounds like both devices try to communicate and then the line is dropped. I want to use the fax in the printer for out-going faxes only, and then use the second fax machine for incoming faxes and faxes that need to be sent manually.
Apple Support, replying to the question:
This configuration is non-standard. Only one fax device should be connected at a time to prevent a carrier signal from activating the second device. Even though the printer has been set to answer after eight (8) rings, it probably senses an ‘off hook’ condition when the standard fax machine answers and then detects the carrier tone causing both fax devices to respond. When neither device can connect properly they both hang up.
We are not aware of a PostScript solution for preventing this problem, however there is a third-party device which may work. The device is not intended for this exact situation, but it should work fine. A company called Command Communications Incorporated produces a line of products called Comshare which allow a single phone line to automatically switch between fax, modem, answering devices, and voice modes. By connecting the standard fax to the fax port and the printer fax to the modem or voice port the problem should be resolved. The Comshare device intercepts incoming calls when the phone rings and listens for signals that are characteristic of a fax, modem, or voice call. It then quickly connects the appropriate device.
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