Why do I get ‘no dial tone’ from my computer even though the phone has a clear dial tone.
The most common way this happens is that the computer’s modem is simply not
connected to the phone line. The dial tone is the modem’s only real way of
knowing whether there’s a phone line there or not.
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Make sure the phone line is plugged into the correct socket in your modem,
or into the PC if the modem is built-in.