Multi-Line Phone Operation - Tandy DCM-7 Operation Manual

Troubleshooting
Multi-line phone operation
The DCM-7 Modem has an internal configuration option which allows the
Modem to be used with multi-line telephones. This option must be performed
by a qualified Radio Shack service technician.
With this option, the internal circuitry can be configured to signal a local
network that there is a MODEM on the line instead of a telephone. Unless
the Modem is modified internally, the local network will start flashing the
BUSY indicator on the multi-line telephone when the Modem seizes the line.
56