Many modems have a switch, jumper, or command thay causes it to connect the yellow and black wires in a telephone jack. This feature is provided so that modems being used with a key switch multi-line office PBX will provide the correct signals to indicate that the modem line is in use and should not be picked-up at another phone.
The following commands, switches or jumpers will disable the modem from shorting the two wires normally used by PhoneNET connectors. Doing the opposite will cause the modem to indicate that the line is in use on multi-line key switch telephone systems.
Hayes Smartmodem 2400: issue commands AT&J0 or AT&J
Hayes Smartmodem 1200: set switch 7 UP
Avatek 1200: only uses two wires
US Robotics Password 1200: pull up jumper J1
US Robotics Courier 2400: set switch 7 UP
Apple Modem 300/1200: pull up jumper J1
Apple Personal Modem: (made in USA) cannot be disabled, cut the yellow
and black wires in the telephone extension cable.
Apple Personal Modem: (made in Hong Kong) only uses two wires
Note: Apple Computer, Inc. is not responsible for the contents of this article.