HP Compaq Series Hardware Manual page 74

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CAUTION: Jacks for digital PBX systems might resemble analog
telephone jacks but are not compatible with the modem.
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CAUTION: Built-in modems may not work with multiple phone lines or a
private branch exchange (PBX), cannot be connected to a coin-operated
telephone, and do not work with party lines. Some of these connections
may result in excess electrical voltage and could cause a malfunction in
the internal modem. Check your telephone line type prior to connecting
your phone line.
If the modem cable contains noise suppression circuitry, which
prevents interference from TV and radio reception, orient the
circuitry end of the cable toward the notebook.
Hardware Guide
Communication Devices
6–2