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When you troubleshoot communications problems in Windows, an MS-DOS-level communications program is often required to test the modem and ports outside of Windows.
If an MS-DOS-based application cannot be used, it is possible to test the modem connections by typing the following command at the MS-DOS command prompt
Echo ATDT > ComX
where X is the number of the serial port the modem is connected to or emulating.
The modem should respond with a dial-tone or just a communication signal.
The ATDT command is a signal to the modem that is interpreted as Attention Dial Tone. The first command, Attention, signals to the modem that it is about to receive information, and the Dial Tone command instructs the modem to enable a line to dial out.
NOTE: The modem may not always give a dial tone with this test procedure because a number does not follow the command; however, the modem should give some kind of signal that information was received.