Fax transmissions occur via the phone line...period.

If your friend has only one line, his fax (data) transmissions will coexist with voice on the same line. If your friend does not want his fax interrupted, then he should disable "call waiting" at the very least. That way, if a fax is in progress, a caller will just get a busy signal. Otherwise, the call waiting feature will send a tone that would disrupt the fax.

My printer has fax capability, but I also have more than one line. The printer is configured to send and recieve on that line only, as that line also has call waiting disabled.

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